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MECHANICAL  ENGINEERING 


FOR     BEGINNERS. 


BY 


R.     S.     M'LAREN. 


TlGlftb   TFlumerous    $Uu0tratfons, 


OF   THE 

UNIVERSITY 

OF 


LONDON: 

CHARLES    GRIFFIN    &    COMPANY,    LIMITED  ; 

EXETER   STREET,    STRAND. 

1908. 

[All  Rights  Reserved.] 


PREFACE. 


OF  books  dealing  with  the  various  branches  of  Mechanical 
Engineering  there  is  an  immense  choice.  Reliable  text- 
books can  be  found  dealing  with  almost  any  subject  upon 
which  an  engineer  can  desire  information ;  but  when 
asked  by  a  beginner,  say  by  a  youth  whose  friends  have 
just  placed  him  as  an  apprentice  or  pupil  with  a  firm  of 
mechanical  engineers,  to  recommend  an  inexpensive  and 
up-to-date  book  on  engineering,  one  finds  some  difficulty 
in  making  a  selection. 

To  the  author  it  appears  that  what  a  beginner  really 
needs  is  a  book  which,  while  giving  in  broad  outlines 
the  information  it  is  necessary  to  possess  concerning  the 
ordinary  branches  of  mechanical  engineering,  yet  shall 
go  sufficiently  into  detail  to  enable  him  to  make  all  the 
calculations  likely  to  be  required  during  the  earlier  stages 
of  his  career. 

It  has  been  the  aim  of  the  author  in  the  following 
pages  to  state  in  clear  language  some  of  the  elementary 
facts  connected  with  mechanical  engineering,  and  to  show 
how  the  simple  calculations  which  have  to  be  made  from 
time  to  time  by  every  engineer  and  draughtsman  can  be 
performed. 

Theory  is  introduced  in  places  where  its  bearing  on 
practice  can  be  seen  and  understood.  For  instance,  the 
theory  of  raising  steam  is  dealt  with  after  the  reader  has 
been  introduced  to  the  various  types  of  boiler  in  use  and 
has  learnt  something  of  the  merits  and  demerits  of  each. 
Again  a  somewhat  important  law  of  Joule  is  not  stated 

203813 


Vi  PEEFACE. 

until  the  Chapter  on  Steam  Turbines  is  reached,  when  the 
student  is  able  to  realise  the  importance  of  the  law  and 
its  application  to  the  De  Laval  turbine. 

The  formulae  given  are  of  the  simplest  character  and 
can  be  worked  out  by  a  beginner  having  no  knowledge 
of  algebra  or  mathematics,  a  knowledge  of  decimals  only 
is  assumed. 

No  attempt  is  made  to  describe  mechanical  processes, 
such  as  turning,  boring,  planing,  or  iron  and  brass 
founding,  as  the  beginner  will  be  able  to  acquire  a  know- 
ledge of  these  processes  in  his  passage  through  the  shops. 
It  is  the  author's  aim  to  give  information  which,  unless 
acquired  by  experience,  can  only  be  obtained  by  reading 
a  considerable  number  of  books  dealing  with  each  subject 
separately. 

In  actual  work  many  practical  questions,  such,  for 
instance,  as  to  the  speed  of  a  centrifugal  pump,  or  of  a 
high-speed  engine,  or  the  heating  surface  of  a  given 
Lancashire  boiler,  frequently  arise.  These  are  usually 
settled  approximately  by  referring  to  the  makers'  cata- 
logues, such  catalogues,  however,  are  not  often  accessible 
to  the  beginner,  and  the  author  hopes  that  the  information 
given  in  this  book,  much  of  which  he  has  found  useful 
in  his  passage  through  the  shops,  drawing  office,  and  at 
the  directors'  table,  may  prove  of  some  service  to  others 
who  are  just  beginning  their  career. 

R.  S.  M'LAREN. 

January,  1908. 


CONTENTS. 


CHAPTER    I. 
Materials. 

PAGES 

Wrought  Iron — Cast  Iron — Malleable  Iron — Steel — Aluminium — 
Copper — Gunmetal — Bronzes  and  Alloys — Testing  Materi- 
als—Stress— Strain— Elastic  Limit— Reduction  of  Area — 
Impact  Tests — Alternate  Bending  Tests — Summary  of 
Weights  and  Strengths — Effect  of  Temperatures  on 
Strength — Fatigue  of  Materials — Factors  of  Safety  — 
Typical  Specifications — Cost  of  Materials, ....  1-18 


CHAPTER    II. 
Bolts  and  Nuts,  Studs,  Set  Screws. 

Bolts  and  Nuts — Studs — Set  Screws — Locking  Devices — Lock 
Nuts — Grover  Washer — Helecoid  Nuts — Castle  Nuts — 
Steady  Pins — Lewis  Bolts — Foundation  Bolts — Table  of 
Whit  worth,  and  Gas  Threads — Rivets — Diameter  and 
Pitch  of  Rivets  for  Various  Joints, 19-26 

CHAPTER    III. 
Boilers. 

Boilers — Cornish  and  Lancashire — Galloway — Economic — Loco- 
motive— Marine — Dry  Back — Vertical- — Dangers  of  the 
Shell  Boiler — Grooving,  Pitting,  and  Wasting — Water- 
tube  Boilers — Babcock — Niclausse — Belleville — Stirling — 
Thornycroft — Yarrow — Strength  of  Boilers  —  Steam  Rais- 
ing— Saturated  and  Superheated  Steam — Evaporation — 
B.T  U.  contained  in  Coal — Rate  of  Combustion — Draught 
of  Chimneys — Area  of  Chimneys— Ratio  of  Grate  Area  to 
Heating  Surface — Evaporation  per  foot  of  Heating  Sur- 
face— Proper  Combustion  of  Coal — Transmission  of  Heat 
through  Boiler  Plates — Feed-water  and  Boiler  Composi- 
tions— Testing  Boilers,  .......  27-56 


Vlll  CONTENTS. 

CHAPTER    IV. 

Steam -Raising-  Accessories. 

PAGES 

Pumps — Duplex — Weir — Deane — Flywheel  Pumps — Inj  ec tors — 
Feed-water  Heaters  and  Economisers — Thermal  Storage 
System  —  Superheaters  —  Mechanical  Stokers  —  Bennis— 
Vicars— Underfeed  and  Chain  Grate— Howden's  Forced 
Draught — Meldrum's  Blower — Coal  Conveying  Plant  — 
Oil  Filters, 57-70 

CHAPTER    V. 
Steam  Pipes  and  Valves. 

Material  —  Arrangement  —  Expansion  —  Size  of  Pipes  —  Flow  of 
Steam  in  Pipes— Radiation  from  Covered  and  Uncovered 
Pipes — Strength  of  Pipes — Size  of  Flanges — Water  Hammer 
— Steam  Traps — Bucket  and  Expansion — Exhaust  Pipes — 
Stop  Valves  —  Hopkinson  -  Ferranti  Valve  —  Isolating 
Valves, 71-88 


CHAPTER    VI. 
The  Steam  Engine. 

Action  Explained— Lap— Lead-  D-Slide  Valve— Piston  Valve- 
Balanced  Slide  Valve— Reversing  Gear— I.H.P.— B.H.P.— 
Method  of  Calculating  Horse-power — Hyperbolic  Curve — 
Reason  why  Single  Cylinder  Engine  Uneconomical — 
Initial  Condensation  —  Compound-  and  Triple-expansion 
Engines — Consumption  of — Corliss,  Willans,  Belliss  and 
Browett-Lindley  Engines — Merits  and  Demerits  of  various 
Types — Steam  Jacketing — Calculating  H.P.  of  Compound 
Engines — Testing  Engines — Brakes — Indicating  an  Engine 
and  Method  of  Working  out  Diagrams,  ....  89-127 


CHAPTER    VII. 
The  Steam  Engine. 

Flywheel  Calculations — Stored  Energy — Stress  in  Rim — Throttle 
and  Expansion  Governors  —  Proportion  of  Engine  Parts 
and  Stresses  —  Engine  Packings  —  Hemp,  Asbestos,  and 
Metallic — Piston  Rings  and  Springs — Efficiency  of  Engines 
— Zeuner  Diagram,  ........  129-147 


CONTENTS.  IX 

CHAPTER  VIII. 
Power  Transmission. 

PAGES 

Rules  for  Power  Transmitted  by  Belts— Speed  of  Belts— Com- 
pounding Belts — Pulleys — Balancing  and  Convexity  of 
Pulleys — Speed  of  Driving  and  Driven  Shafts — Thickness 
of  Belts  for  Small  Pulleys— Fast  and  Loose  Pulleys- 
Ropes,  Rules  for  Power  Transmitted  by — Speed  of  Ropes 
— Size  of  Rope  Pulleys  —  Distance  Apart  of  Pulleys — 
Clutches  for  Rope  Pulleys — Loss  of  Power  in  Transmission 
— Shafting,  Rules  for  Power  Transmitted  by — Bearings 
— Gearing  —  Toothed  Wheels  —  Helical  Wheels  —  Worm 
Wheels— Skew  Wheels— Raw- hide  Pinions— Hans  Rey- 
nolds' Silent  Chain -Rules  for  Power  Transmitted  by 
Gearing, 149-163 

CHAPTER  IX. 
Condensing  Plant. 

Object  of  Condensing  Plant — Jet  Condenser — Amount  of  Cooling 
Water  Required — Air  Pumps — Surface  Condenser — Extent 
of  Cooling  Surface — Amount  of  Cooling  Water  Required — 
Capacity  of  Air  Pump — Temperatures  and  Corresponding 
Vacua— Dry  and  Wet  Air  Pumps — Vacuum  Augmenter — 
Corrosion  in  Condenser  Tubes — Edwards'  Air  Pump — 
Evaporative  Condensers — Ejector  Condenser  —  Barometric 
Condenser — Cooling  Towers,  ......  165-176 


CHAPTER  X. 
The  Steam  Turbine. 

Parsons  Turbine  — Willans-Parsons  Turbine -Brush-Parsons  Tur- 
bine—  Speeds  and  Outputs  —  De  Laval  Turbine — Curtis 
Turbine— Rateau  Turbine — Westinghouse  Turbine — Zoelly 
Turbine— General  Remarks, 177-192 


CHAPTER  XL 
Electrical  Chapter. 

Production  of  Current  —  Magnetic  Field  —  E.  M.  F.  —  Simple 
Dynamo — Volts,  Amperes,  Ohms,  and  Watts — Electrical 
Horse-power — Power  and  Current  Required  for  Lamps — 
Reason  for  High  Voltages — Alternators — Transformers — 
Single  and  Multiphase  Currents — Series-wound,  Compound- 
wound,  and  Shunt-wound  Dynamos — Rotary  Converters — 
Motor  Generators — Primary  Batteries  and  Accumulators — 
Summary  of  Electrical  Terms— Table  of  Conductors,  .  193-209 


X  CONTENTS. 

CHAPTER  XII. 
Hydraulic  Machinery. 

PAGES 

Hydraulic  Press  and  Hand  Pump— Hydraulic  Accumulator — 
Pressures  Used  by  Hydraulic  Engineers  —  Hydraulic 
Riveters,  Fixed  and  Portable — Flanging  Press — Lifts  and 
Cranes  —  Jiggers — Jack — Water  Wheels — Overshot,  Under- 
shot, and  Breast  Wheels — Turbines — Parallel  Flow — Radial 
Flow — Mixed  Flow — Advantages  and  Disadvantages  of 
each  —  Impulse  Wheels  —  Power  obtained  from  Falling 
Water — Suction  Tube  —  Governing — Bearings  —  Niagara 
Falls  Turbines — Falls  of  Foyers  Turbines — Pumps-  Cen- 
trifugal—Multiple-Stage Pumps — Sizes  and  Speeds  — 
Reciprocating  Pumps — Pumping  Hot  Water — Valves — 
Pulsometer  Pump — Hydraulic  Ram — Materials  for  Pump- 
ing Various  Liquids — Flow  of  Water  in  Pipes  and  Useful 
Memoranda, 211-237 

CHAPTER  XIII. 
Gas  and  Oil  Engines. 

Otto  Cycle  —  Description  of  10-H.P.  Engine— Methods  of  Ignition 
— Exhausting  and  Scavenging— Drawing  in  Mixture  —  Com- 
pression—  Exceptions  to  Otto  Cycle — Korting  Engine — 
Amount  of  Gas  Consumed  and  B.T.U.  Contained  in  Gas — 
Horse-power  of  Gas  Engines — Temperatures  and  Pressures 
Reached — Speeds  —  Thermal  Efficiency — Distribution  of 
Heat — Suction  Gas  Plant — Amount  of  Gas  Made  and 
Water  Required  —  Starting  Gas  Engines — Comparative 
Merits  of  Steam  and  Gas  Engines — Oil  Engines — Con- 
sumption of  Oil  and  Petrol, 239-254 

CHAPTER   XIV. 
Strength  of  Beams  and  Useful  Information. 

Strength  of  Beams  having  Top  and  Bottom  Flanges — Strength  of 
Rectangular  Beams — Table  of  Rolled-steel  Joists — Ascer- 
taining Stresses  by  Graphic  Methods — Calculating  Power 
Transmitted  by  Screws,  Levers,  and  Wedges — Thermo- 
meter Scales — To  Divide  a  Straight  Line  into  a  Number 
of  Equal  Parts  —  Decimal  Equivalents  of  an  Inch  with 
Areas  and  Circumferences— Table  of  Areas  and  Circum- 
ferences —  Table  of  Squares  and  Cubes  —  Weights  and 
Measures  with  Metrical  Equivalents,  ....  255-269 

CHAPTER   XV. 
Conclusion, 271-276 

INDEX,  .  277 


LIST    OF    ILLUSTRATIONS. 


PIGS.  PAGE 

1.  Incorrect  method  of  ribbing  a  flat  plate,        .         .         .         .11 

2.  Correct  „              „              ,,                 ....       11 

3.  Bolt  and  nut, 19 

4.  Stud, 19 

5.  Set/  screw, .       19 

6.  Lock  nuts,      .         .         .         .         . 20 

7.  Grover  washer,       .         .         .         ....         .         .         .20 

8.  Castle  nut, 20 

9.  Helicoid  nut,  .........       20 

10.  Set  screw  and  steady  pin, 20 

11.  Lewis  bolt,     .         .         .         .         .  .         .         .         .20 

12.  Foundation  bolt  and  anchor  plate,         .         .         .         .         .21 
13,  14.     Rivets,  before  and  after  closing, 24 

15.  Lancashire  boiler,  .........       28 

16.  ,,  front  view  with  setting,    ....       29 

17.  Cornish  boiler,  front  view, 32 

18.  Locomotive  boiler,  ........       34 

19.  Marine  ,,....                                           35 
19a.  -Blake"          „               36 

20.  Babcock  ,,              ........       38 

21.  Worthington  Duplex  pump,  .......       57 

21a.  Weir  feed  pump,    .         .         .         ...         .         .         .59 

22.  Cameron  feed  pump,       .......  60 

23.  Diagram  showing  action  of  injector,       .....       62 

24.  Modern  injector, 63 

25.  Feed-water  heater,          .......  64 

26.  Underfeed  stoker, !       68 

27.  Dangerous  pipe  arrangement,         ......       72 

28.  Correct  position  of  stop  valve  on  boiler,        ....       73 

29.  Incorrect  ,,             ,,             ,,                    ....       73 

30.  Stop-valve  passage  obstructed  by  water,        .         .         .         !       77 

31.  Bucket  steam  trap,         ......  83 

32.  Expansion         ,,  .         .         .         .         .         ,         t                 $3 

33.  Screw-down  stop  valve, !       85 

34.  Straight-through  gate  stop  valve,  ...                         86 

35.  Vertical  single-cylinder  steam  engine,  .....       90 

36.  Cylinder  with  piston  valve, '.92 

37.  Reversing  gear, !       92 

37a.  Setting  eccentrics,          .         .         .         .         .         .         .         .92 

38.  L.P.  cylinder  with  balanced  slide  valve,        .         .         '.         !       93 
39,  40,  41.     Hyperbolic  curves,  .....                                  99 

42.  Corliss  gear, .         !         .     105 

43.  L.P.  cylinder  with  Corliss  valves, 105 

44.  Drop-valve  engine,         .         .         .         .         ...         .'     107 

44a.  Van  den  Kerchove  engine,      .......     107 

45.  Willans  triple-expansion  engine,    ....'.  109 


Xil  LIST    OP    ILLUSTRATIONS. 

FIGS.  PAGE 

46.  Crosby  indicator, .         .         .121 

46a.         „  „         for  superheated  steam,        .         .         .         .121 

47.  H.  P.  Indicator  diagram 124 

48.  L.P.         ,,  „  124 

49.  Metallic  packing, 141 

50,  51,  52.     Ramsbottom  piston  rings, 143 

53,  54,  55.     Willans  piston  rings, 143 

56,  57.     Mudd  piston  rings, 143 

58.  Zeuner  diagram,     .........     146 

59.  ,,  with  piston  and  cylinder,    .        .        .        .146 
60,61.  Worm  and  wheel, .         ........     100 

62.  Hans  Reynolds'  chain, 162 

63.  Jet  condenser,        .         .         .         .         .         .         .         .         .166 

64.  Air  pump,       ..........     167 

65.  Surface  condenser, 168 

66.  Condenser  tube  end  and  ferrule, 169 

67,  68.     Edwards'  air  pump, 172 

69.  Willans-Parsons  steam  turbine, 178 

70.  Turbine  blading 179 

71.  „       baffle  rings 179 

72.  Method  of  strengthening  Parson's  blades,      .         .         .         .179 

73.  Bucket  wheel  of  De  Laval  turbine, 183 

74.  De  Laval  turbine  with  gearing,      .         .         .         .         .         .184 

75.  Curtis  turbine  blading, 186 

76.  „  187 

77.  Horse-shoe  magnet, 1 94 

77a.  „  „  194 

78.  Direct  current  12-pole  dynamo, 199 

79.  Alternator, 201 

80,  81,  82,  83.     Alternating-current  diagrams, 202 

84.  „  „  „  203 

85.  ,,  „        generator, 204 

86.  Hydraulic  press, 212 

87.  Portable  hydraulic  riveter, 212 

88.  Hydraulic  U  leather, 212 

89.  accumulator, 213 

90.  riveter, 215 

91.  .  flanging  press, 218 

92.  punching  machine .218 

93.  jigger, 219 

94.  jack, 219 

95.  Water  turbine,  Jonval  type, 222 

96.  „  radial  inward-flow  type 223 

97.  „  ,,       outward-flow  type,      .        .        .        .224 

98.  Centrifugal  pump, 229 

99.  Pulsometer      ,, 233 

100.  Gas  engine, 240 

101.  Korting  gas  engine,        ........     243 

102.  Suction  gas  producer, 248 

103.  Hornsby  oil  engine,        ........     253 

104,105,106.     Graphic  methods  of  computing  stresses,      .        .         .260 


MECHANICAL    ENGINEERING 

FOR    BEGINNERS. 


CHAPTER    I. 
MATERIALS. 

THE  materials  chiefly  used  in  mechanical  engineering  are — 
wrought  iron,  cast  iron,  steel,  aluminium,  gunmetal,  and 
kindred  alloys.  Iron  may,  for  practical  purposes,  be  divided 
into  two  distinct  classes — viz.,  wrought  and  cast — and  although 
the  difference  between  them  lies  principally  in  the  fact  that 
there  is  less  carbon  and  silicon  in  wrought  iron  than  in  cast 
iron,  yet  in  practice  the  distinction  between  the  two  forms  of 
iron  is  so  marked  that  they  might  almost  be  two  distinct  metals. 

Wrought  Iron  is  fibrous,  can  be  bent  (or  given  a  permanent 
set)  without  breaking ;  it  can  be  sheared  and  punched ;  when 
hot  it  can  be  worked  under  the  hammer,  and  forged  into  various 
shapes.  Two  pieces  of  this  metal  can  be  joined  or  welded  to- 
gether by  heating  them  to  what  is  known  as  welding  heat  (about 
1,600°  F.)  and  hammering  the  two  parts  together.  The  surface 
of  wrought  iron  can  be  made  exceedingly  hard,  if  required,  by 
case-hardening.  This  is  effected  by  surrounding  the  metal  with 
cuttings  of  leather,  horn,  and  bone  dust,  raising  the  whole  to  a 
considerable  heat,  maintaining  the  heat  for  some  hours,  and  then 
slowly  cooling.  The  carbon  from  these  chippings  enters  into  the 
composition  of  the  metal,  and  forms  a  surface  closely  allied  to 
steel.  Case-hardening  can  also  be  effected  by  heating  the  metal 
to  a  bright  red  and  applying  prussiate  of  potash,  allowing  to  cool 
slightly,  and  then  plunging  into  cold  water.  This  method  is 
quicker  than  the  one  previously  mentioned,  but  is  more  liable 
to  cause  distortion. 

Wrought  iron  is  used  in  cases  where  considerable  strength  is 
necessary,  and  the  required  form  is  a  simple  one,  such  as  can  be 
obtained  by  forging,  welding,  and  machining  the  metal,  stamping 
out  of  the  solid,  or  where  the  structure  can  be  built  up  of  plates, 
Angle  and  Tee  irons.  During  recent  years  wrought  iron  has 


2  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

been  almost  entirely  superseded  in  mechanical  engineers'  works 
by  mild  steel,  but  the  former  is  still  used  in  cases  where  it 
requires  to  be  welded,  as  in  chains,  tubes,  eye-bolts,  &c.  In 
cases  where  shocks  have  to  be  withstood,  wrought  iron  is  superior 
to  mild  steel,  as  the  former  has  not  the  same  tendency  to  crystal- 
lise or  lose  its  fibrous  character.  The  coupling  links  of  railway 
waggons  are  always  made  of  the  best  wrought  iron. 

Wrought  iron  and  mild  steel  are  brought  into  an  engineer's 
works  in  the  following  forms,  viz.: — Round,  square,  and  flat 
bars  ;  sheets  or  plates,  Tee,  Angle,  and  Channel  iron.  The 
metal  is  formed  into  these  shapes  by  being  passed  through  rolls 
when  hot. 

Wrought  iron  varies  considerably  in  quality ;  its  distinguishing 
marks,  in  addition  to  the  maker's  brand,  are — B,  BB,  and  BBB ; 
the  letter  B  signifying  the  word  "  best." 

Wrought  iron  of  good  quality  will  safely  bear  a  stress  of  5  tons 
per  square  inch  in  simple  tension — that  is  to  say,  a  bar  1  inch 
square  will  safely  support  any  weight,  up  to  5  tons,  suspended 
from  it.  If,  however,  the  load  is  likely  to  be  a  varying  one,  say 
from  0  to  the  maximum,  then  the  bar  should  not  be  loaded  to 
more  than  3|  tons  per  square  inch.  The  breaking  stress  of  the 
best  Yorkshire  bars  in  simple  tension  is  about  28  tons  per  square 
inch,  and  of  Staffordshire  bars  about  25  tons  per  square  inch. 
The  elongation  before  fracture  varies  from  8  to  30  per  cent, 
on  a  test  piece  10  inches  long.  Wrought  iron  will  safely  bear 
a  stress  of  4  tons  in  compression — i.e.,  a  block  1  inch  square 
will  safely  support  a  weight  of  4  tons  placed  upon  it.  Wrought 
iron  will  begin  to  crush,  or  give,  under  a  pressure  of  13  to 
20  tons  per  square  inch.  If  subjected  alternately  to  tension 
and  compression,  wrought  iron  should  not  be  loaded  to  more 
than  2  tons  per  square  inch. 

A  cubic  inch  of  wrought  iron  weighs  approximately  '28  Ib. 
The  weight  of  any  piece  or  structure  of  wrought  iron  is,  there- 
fore, easily  ascertained  by  finding  the  number  of  cubic  inches  it 
contains  and  multiplying  by  -28. 

Wrought  iron  melts  at  a  temperature  of  from  2,700°  to 
2,920°  F.*  It  contains  not  more  than  -2  per  cent,  of  carbon. 

Cast  Iron  is  granular  and  of  a  brittle  nature ;  it  cannot  be 
bent,  sheared,  or  punched,  and  is  liable  to  break  suddenly  if  too 

*  The  melting  point  of  metals  is  not  easy  to  determine  with  great 
accuracy.  Mercury  boils  at  680°  F.,  and,  therefore,  cannot  be  used. 
Very  high  temperatures  are  usually  determined  by  noting  the  resistance 
caused  to  an  electric  current  when  passed  through  a  platinum  wire,  the 
latter  being  placed  in  a  porcelain  or  quartz  casing.  The  higher  the 
temperature  the  greater  the  resistance. 


MATERIALS.  6 

great  stress  is  put  upon  it.  Cast  iron,  however,  can  be  run, 
when  in  a  molten  state,  into  intricate  shapes.  Two  pieces  of 
this  metal,  after  working  together  for  some  time  in  sliding  con- 
tact, acquire  very  smooth  surfaces,  and  the  amount  of  friction 
between  them  is  less  than  with  any  other  two  similar  pieces  of 
metal. 

Cast  iron  is  used  in  all  cases  where  an  intricate  form  is  required 
and  sufficient  metal  can  conveniently  be  given  to  withstand  the 
stresses  likely  to  come  upon  it,  and  where  corrosion  can  be 
prevented  by  the  friction  of  the  parts  or  by  other  methods.  Cast 
iron  is  used  for  steam  cylinders,  bedplates,  and  frames  of  various 
machines,  for  flywheels,  large  water  pipes,  railway  chairs,  and 
for  an  enormous  variety  of  purposes.  Cast  iron  is  brought  into 
an  engineer's  foundry  in  the  form  of  pig  iron :  when  required  for 
use  it  is  broken  into  smaller  pieces  and  melted  down,  together 
with  a  certain  proportion  of  old  broken  cast  iron  (scrap),  when 
this  can  be  obtained,  in  a  cupola. 

The  cupola  consists  of  an  upright  wrought-iron  casing  lined 
with  fire-brick,  fireclay,  and  a  fire-resisting  substance  called 
Ganister.  The  fire  having  been  kindled,  the  cupola  is  charged 
with  coke  and  pig  iron  (broken)  in  alternate  layers,  and  a  blast 
of  air  from  a  fan  or  blower  is  driven  in,  the  iron  melts  and 
trickles  down  through  the  coke  to  the  bottom  of  the  cupola.  It 
is  then  drawn  off  in  a  liquid  state  and  poured  into  moulds  made 
to  any  desired  shape  in  sand.  While  the  first  charge  is  being 
drawn  off  and  used,  a  further  supply  of  coke  and  iron  is  thrown 
in  at  an  opening  at  the  top  of  the  cupola. 

Pig  iron  is  obtained  in  differing  degrees  of  hardness  and 
strength ;  these  degrees  are  known  by  numbers.  No.  1  is  very 
soft  and  easy  to  machine,  but  is  deficient  in  strength,  and  is  to  a 
certain  extent  porous.  No.  2  is  slightly  harder  than  No.  1. 
No.  3  pig  is  hard,  tough,  and  close  grained ;  it  is  largely  used 
for  good  castings.  Nos.  4,  5,  and  6  are  still  harder  and  tougher 
than  the  foregoing.  The  degree  of  hardness  of  old  cast-iron 
scrap  depends  upon  the  quality  of  the  original  metal  and  the 
number  of  times  it  has  been  melted  down.  Scrap  is  cheaper 
than  pig  iron,  but  castings,  if  made  entirely  from  this  source, 
would  be  deficient  in  strength,  very  brittle,  and  extremely  hard 
to  machine. 

The  cast-iron  parts  of  machines  that  are  subject  to  wear,  such 
as  steam  cylinders  (in  which  the  piston  moves  at  a  considerable 
speed),  should  be  made  much  harder  than  is  necessary  for  those 
parts  which  are  not  subject  to  wear,  such  as  bedplates,  &c.  For 
steam  cylinders  a  certain  proportion  of  No.  4,  and  even  No.  5 
pig,  is  added  to  a  mixture  of  No.  3  pig  and  scrap  iron.  For 


4  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

castings  of  an  inferior  kind  it  is  customary  to  use  a  large  pro- 
portion of  scrap  and  a  very  small  proportion  of  No.  1  pig  iron. 
Every  time,  within  certain  limits,  cast  iron  is  remelted  it  becomes 
harder  and  stronger,  but  more  brittle.  Small  castings  can  be 
made  exceedingly  hard  by  pouring  the  molten  metal  into  iron 
moulds  or  chills  instead  of  into  sand  moulds.  Castings  treated 
in  this  way  are  called  "chilled." 

Cast  iron,  if  of  really  good  metal,  thoroughly  sound,  and  free 
from  internal  stresses  set  up  by  unequal  contraction  in  cooling,, 
will  safely  bear  a  stress  of  1J  to  1J  tons  per  square  inch  in 
simple  tension,  but  in  cases  where  the  stress  varies  with  con- 
siderable frequency  from  0  to  maximum,  the  metal  should  not 
be  loaded  to  more  than  1  or  1 J  tons  per  square  inch.  In  cases 
where  the  metal  is  subjected  to  alternate  tension  and  com- 
pression, the  working  stress  should  not  exceed  J  a  ton  per 
square  inch. 

The  ultimate  or  breaking  tensile  stress  of  cast  iron  varies 
from  8  to  14  tons.  It  will  safely  bear  a  stress  of  6  tons  in 
compression.  The  ultimate  crushing  stress  is  from  25  to  50 
tons  per  square  inch.  A  cubic  inch  weighs  from  -26  to  '27  Ib. 
The  melting  point  of  cast  iron  is  about  2,050°  F.  in  the  case 
of  No.  1  pig.  Nos.  4  and  5  pig  require  a  temperature  of  about 
2,250°  before  melting.  Cast  iron  contains  from  3  to  3£  per  cent, 
of  carbon  and  from  1J  to  2^  per  cent,  of  silicon.  Not  more 
than  1 J  per  cent,  of  the  carbon  is  combined  with  the  iron,  the 
remainder  is  present  in  the  form  of  graphite. 

The  transverse  strength  of  cast  iron  is  usually  ascertained  by 
placing  a  bar,  2  inches  deep  by  1  inch  wide,  upon  supports  36 
inches  apart,  and  loading  it  in  the  centre.  A  bar  tested  in  this 
way  will  break  with  a  load  of  from  25  to  40  cwts.,  depending 
upon  the  quality  of  the  iron.  In  such  a  test  the  molecules  in 
the  upper  part  of  the  bar  are  in  compression,  while  those  in  the 
lower  part  are  in  tension.  A  typical  specification  is  given  at  the 
end  of  the  chapter. 

Malleable  Cast  Iron  is  not  so  brittle  as  ordinary  cast  iron ; 
it  will  bend  slightly  before  breaking.  Ordinary  castings  can  be 
made  slightly  malleable  by  surrounding  them  with  a  substance 
which  will  extract  some  of  the  carbon,  such  as  crushed  red 
hematite,  or  black  oxide  of  iron  (iron  scale),  and  placing  them 
in  an  oven  maintained  at  a  temperature  of  1,800°  to  2,000°  F. 
during  the  day  and  at  a  dull  red  heat  during  the  night  for 
several  days  and  nights,  the  length  of  time  depending  upon  the 
size  of  the  casting.  If  the  castings  are  required  to  be  very 
malleable,  they  are  made  of  the  best  Cumberland  white  or  grey 
pig  iron.  Such  castings,  before  being  treated,  are  extremely 


MATERIALS.  5 

brittle.  Malleable  castings  are  used  in  cases  where  shocks  may 
have  to  be  withstood,  and  where  the  form  renders  it  difficult 
or  expensive  to  make  the  article  of  wrought  iron,  and  where 
the  expense  of  gunmetal  or  bronze  has  to  be  avoided. 

Steel. —  In  steel  there  is  not  the  same  hard  and  fast  division 
between  cast  and  wrought  as  there  is  between  cast  and  wrought 
iron,  but  for  the  purpose  of  classification  finished  steel  may  be 
divided  into  three  classes — viz.,  mild,  cast,  and  tool  steel. 

Mild  steel  has  many  of  the  properties  of  wrought  iron,  but  is 
stronger.  It  can  be  bent,  sheared,  forged,  and,  if  it  has  a  very 
small  percentage  of  carbon,  can  be  welded.  Holes  in  steel 
plates,  unless  the  plates  are  very  thin,  should  be  drilled  and  not 
punched,  as  punching  injures  the  surrounding  metal.  Mild  steel 
can  be  cut  or  machined  when  cold  by  tool  steel.  Mild  steel 
usually  enters  an  engineer's  works  in  the  form  of  round  and 
square  bars,  plates,  and  rough  forgings.  It  is  used  in  cases 
where  greater  strength  and  hardness  are  required  than  are 
obtained  with  wrought  iron,  but  where  in  other  respects  the 
latter  would  be  used. 

Mild  steel  is  largely  used  for  crank  shafts,  piston-rods,  eccen- 
tric-rods, boiler  plates,  cross-head  pins,  studs,  rivets,  &c.  Steel 
containing  a  small  proportion  of  nickel  is  stronger  than  ordinary 
mild  steel,  and  does  not  rust  so  easily,  but  is  more  difficult  to 
machine.  Mild  steel  will  safely  bear  a  stress  of  about  6  tons  in 
simple  tension  or  in  compression,  or  4  tons  when  the  load  varies 
from  0  to  maximum,  but  when  subject  alternately  to  tension  and 
compression,  as  in  a  piston-rod  of  a  double-acting  engine,  2J  tons 
per  square  inch  is  a  sufficient  load  if  a  long  life  is  desired.  The 
breaking  tensile  stress  of  mild  steel  is  about  30  tons,  and  the 
elongation  20  to  35  per  cent,  on  10  inches.  The  torsional 
strength  of  mild  steel  is  dealt  with  in  the  chapter  on  transmis- 
sion of  power,  and  some  remarks  as  to  its  ability  to  withstand 
shock  will  be  found  later  on.  The  weight  of  a  cubic  inch  is 
•288  Ib.  Mild  steel  contains  from  -1  to  '5  per  cent,  of  carbon. 
It  does  not  appreciably  harden  if  heated  and  plunged  in  water. 

Cast  Steel  is  harder  and  stronger  than  cast  iron,  and  will 
bend  before  breaking ;  in  fact,  the  best  qualities  are  improved 
by  being  forged.  Great  care  has  to  be  exercised  in  making 
steel  castings,  as  the  metal  in  cooling  gives  off  a  gas,  which, 
unless  got  rid  of,  honeycombs  the  casting  with  small  holes. 
Molten  steel  is  not  so  fluid  as  iron,  and  does  not  fill  the 
cavities  so  completely  as  the  latter.  Messrs.  Whitworth  cast 
some  of  their  steel  under  pressure  in  order  to  overcome  these 
difficulties. 

Cast  steel  is  used  in  cases  where  great  strength  is  required, 


6  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

and  where  the  form  is  only  moderately  intricate.  It  is  used  for 
toothed  wheels,  hydraulic  cylinders,  high-pressure  valve  bodies, 
for  the  arms  of  hydraulic  riveters,  &c.  Steel  castings,  if  sound, 
will  safely  bear  a  stress  of  from  4  to  6  tons  per  square  inch 
in  simple  tension  or  compression.  The  breaking  tensile  stress 
is  from  20  to  30  tons,  with  an  elongation  of  15  to  25  per  cent, 
on  2  inches.  Whitworth  fluid- com  pressed  steel  has  an  ultimate 
breaking  strength  of  about  40  tons  per  square  inch,  with  an 
elongation  of  about  32  per  cent.  The  weight  of  a  cubic  inch 
is  -288  Ib.  Steel  castings  usually  contain  about  *251  per  cent, 
of  carbon.  The  melting  point  is  from  2,400°  to  2,600°  F. 

Tool  steel  is  much  harder  than  mild  steel,  and  is  more  difficult 
to  forge.  Tool  steel  usually  enters  an  engineer's  works  in  square, 
hexagonal,  or  round  bars ;  it  is  cut  and  forged  to  the  required 
shape  and  size,  and  is  then  hardened  and  tempered  to  make  it 
sufficiently  hard  to  cut  mild  steel,  cast  iron,  or  wrought  iron. 

Hardening  is  effected  by  heating  the  piece  of  steel  to  a  cherry 
red  (1,600°  to  1,800°  F.)  and  then  plunging  it  into  cold  water 
or  oil,  A  piece  of  steel  treated  in  this  way  only  would  be  too 
hard  and  brittle  for  many  purposes.  It  is  tempered  by  being 
again  heated  to  a  temperature  of  between  430°  and  570°  F.  and 
then  suddenly  plunged  into  cold  water.  If  the  piece  of  steel  is 
of  adequate  size  the  hardening  and  tempering  can  be  done  at 
one  heating.  The  tool  is  heated  to  a  cherry  red,  the  part  to 
be  hardened  only  is  plunged  into  water  and  cooled ;  sufficient 
heat  then  travels  along  the  steel  from  the  uncooled  portion  to 
raise  the  temperature  of  the  cooled  part  to  the  required  degree, 
when  the  whole  piece  is  plunged  in  water.  The  temperature  is 
known  by  the  colour  of  a  film  of  oxide  which  appears  on  the 
bright  steel. 

Self -hardening  Tool  Steel. — If  a  tool,  after  being  hardened 
and  tempered,  is  reheated  and  allowed  to  cool  slowly  it  loses 
its  temper  and  becomes  soft.  When  a  tool  is  used  for  cutting 
hard  materials  at  a  high  speed  the  tool  becomes  hot,  and  if  the 
heat  reaches  a  certain  point  the  tool  loses  its  temper.  It  was 
found,  however,  that  by  adding  tungsten  and  a  small  percentage 
of  manganese  and  chromium  the  steel  became  self-hardening — 
that  is  to  say,  it  did  not  become  soft  when  heated  and  allowed 
to  cool.  Mushet  steel  was  the  best  known  example  of  this  class 
of  steel. 

High-speed  Tool  Steel. — A  still  further  advance  in  steel  used 
for  cutting  purposes  has  been  made  in  what  is  known  as  high- 
speed tool  steel.  In  it  the  percentages  of  tungsten  and  chromium 
are  increased,  and  that  of  the  carbon  diminished.  The  steel  is 
heated  almost  to  melting  point,  then  cooled  by  an  air  blast ;  then 


MATERIALS.  7 

reheated  to  a  dull  red  and  again  cooled  by  an  air  blast.  In 
hardening  and  tempering  these  steels  in  the  works,  the  maker's 
directions,  which  are  sent  out  with  every  bar,  must  be  followed. 
Quenching  in  water  is  forbidden. 

The  percentage  of  carbon  in  tool  steel  varies  considerably.  In 
ordinary  cutting  tool  steel  there  is  about  1  per  cent. ;  in  Mushet 
steel  about  2-15  per  cent. ;  in  high  speed  tool  steel  about  from 
1  '25  down  to  *6  per  cent.  The  ultimate  tensile  strength  of  tool 
steel  is  from  40  to  60  tons  per  square  inch.  The  elongation  is  5 
to  12  per  cent,  on  2  inches. 

As  steel  is  often  referred  to  as  being  of  the  Bessemer  or  open 
hearth  process,  it  may  be  well  to  state  briefly  what  these 
processes  are.  Steel  is  obtained  from  pig  iron  by  removing  a 
large  proportion  of  its  carbon,  and  as  much  of  the  silicon, 
phosphorus,  and  sulphur  as  possible.  In  the  Bessemer  process 
this  is  effected  by  blowing  air  through  the  molten  pig  placed  in 
a  converter ;  as  a  result  the  silicon  is  first  burnt  out  and  forms 
slag,  which  is  removed ;  the  carbon  then  burns,  making  a  long 
flame  of  carbon  monoxide ;  when  all  the  carbon  is  burnt  the 
flame  ceases  and  a  certain  amount  of  Ferro-manganese  or  Spiegel- 
eisen  is  added  to  give  the  required  percentage  of  carbon  to  the 
steel. 

In  the  Siemens-Martin  open  hearth  process,  pig  iron  and 
wrought-iron  scrap  are  melted  together  and  a  proportion  of 
hematite  added ;  the  air  is  not  blown  through  the  metal  as  in 
the  Bessemer  process,  but  a  mixture  of  air  and  producer  gas  are 
burnt  above  the  molten  metal.  As  the  pig  iron  is  mixed  with 
scrap  iron  the  amount  of  silicon  and  carbon  is  not  so  great  as  in 
a  corresponding  quantity  of  pig  iron  alone,  and  what  silicon  and 
carbon  there  is  is  got  rid  of  by  oxidation  caused  by  the  burning 
gases,  aided  by  the  oxygen  of  the  hematite.  If  the  whole  of  the 
carbon  is  removed  a  certain  amount  of  ferro-manganese  is  added 
as  in  the  Bessemer  process. 

The  Siemens  open  hearth  process  is  similar  to  the  Siemens- 
Martin,  but  in  the  former  pure  pig  iron  and  not  pig  and  scrap  is 
used. 

Briefly,  it  may  be  stated  that  the  Bessemer  process  is  chiefly 
used  for  making  high  carbon  steel,  and  the  open  hearth  process 
for  mild  steel. 

One  frequently  reads  of  "acid"  and  "basic"  steel;  these 
terms  mean  that  the  lining  of  the  open  hearth  furnace  or 
converter  was  made  either  of  an  acid  or  basic  material.  The 
lining  of  the  furnace  has  a  certain  effect  in  removing  impurities 
from  the  steel. 

Steel  which  contains  phosphorus  is  brittle  and  is  thought  to 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

deteriorate  in  time,  especially  if  subjected  to  stresses  of  an 
alternating  kind. 

A  small  amount  of  nickel,  as  already  stated,  increases  the 
tenacity  of  steel.  A  small  percentage,  too,  of  vanadium  has  a 
beneficial  effect ;  it  appears  to  cause  the  carbon  to  distribute  itself 
more  evenly,  and  adds  to  the  ability  of  the  steel  to  resist  shock. 

Aluminium  is  a  metal  which  is  used  in  cases  where  lightness 
is  required  ;  it  can  be  cast  into  intricate  shapes.  It  is  very  soft, 
but  has  a  fair  tensile  strength,  approximately  that  of  cast  iron. 
Its  ultimate  breaking  strength  in  simple  tension  is  about  8  tons 
per  square  inch,  and  may  in  practice  be  loaded  to  1  ton  per 
square  inch.  An  alloy  formed  of  aluminium  and  zinc  is  still 
stronger.  Aluminium  bronze  is  dealt  with  under  alloys.  A 
cubic  inch  weighs  *09  Ib.  The  melting  point  is  about  1,200°  F. 

Copper  is  fibrous  and  very  soft ;  it  can  be  bent,  sheared,  or 
hammered  into  various  shapes  when  cold.  After  hammering  it 
requires  to  be  annealed — i.e.,  raised  to  a  high  temperature  and 
then  suddenly  cooled.*  Copper  is  a  good  conductor  of  heat  and 
does  not  readily  corrode.  Two  pieces  of  copper  can  be  joined 
together  by  brazing  :  brazing  is  effected  thus — the  two  pieces  to 
be  united  are  placed  together,  brazing  metal  in  the  form  of 
borings  is  placed  over  the  joint  and  the  whole  is  covered  with 
powdered  borax ;  the  seam  is  then  held  over  a  coke  fire  or 
gas  flame  until  the  brazing  metal  melts  and  unites  the  two 
pieces  of  copper.  Borax,  which  acts  as  a  flux,  is  thrown  on 
during  the  operation.  A  joint  which  has  been  properly  brazed 
is  as  strong  as  the  metal  itself. 

Copper,  on  account  of  its  heat-conducting  properties,  is  used 
in  the  construction  of  locomotive  fire  boxes.  It  is  also  an 
extremely  good  conductor  of  electricity  and  is  largely  used  by 
electrical  engineers.  Copper  is  also  used  for  small  pipes  which 
require  to  be  bent  cold. 

Copper  will  safely  bear  a  tensile  stress  of  1 J  tons  per  square 
inch  at  temperatures  below  300°  F. ;  above  this  temperature  the 
strength  rapidly  falls  off.  The  breaking  tensile  stress  of  copper 
bolts  and  plates  at  300°  F.  is  about  14  tons  per  square  inch,  with 
an  elongation  of  about  35  per  cent,  on  2  inches.  At  600°  F.  the 
strength  falls  to  about  10  tons  per  square  inch.  The  melting 
point  of  copper  is  about  1,950°  F.  If  the  heat-conductivity  of 
gold  is  taken  as  100,  that  of  copper  is  about  87,  while  iron  or 
steel  has  a  heat-conductivity  of  about  37  only.  A  cubic  inch 
weighs  -322  Ib. 

Qunmetal,  Bronzes,  and  Kindred  Alloys. — Gunmetal  is 

*  When  steel  is  annealed  it  is  raised  to  a  high  temperature  and  slowly 
cooled. 


MATERIALS. 

stronger  than  cast  iron  and  will  bend  slightly  before  breaking. 
It  is  soft  and  easy  to  machine,  can  be  cast  into  intricate 
shapes,  is  not  porous,  and  does  not  readily  corrode,  but  is  ex- 
pensive ;  it  cannot  be  forged  or  welded.  Gunmetal  is  used  in 
cases  where  comparatively  small  castings  are  required  to  be  of 
an  intricate  form  and  of  moderate  strength,  also  in  cases  where 
corrosion  has  to  be  avoided.  It  is  used  largely  for  the  internal 
parts  of  steam  and  water  valves,  for  cocks,  and  engine  fittings. 

Gunmetal  is  largely  used  for  bearings  in  which  steel  shafts 
revolve,  as  the  friction  between  this  alloy  and  steel  is  not 
excessive,  and  gunmetal,  being  the  softer,  wears  away  before  the 
steel:  when  worn  to  too  great  an  extent  the  "brasses"  can  be 
replaced.  Bearings  that  are  subject  to  heavy  and  continuous 
loads  are  usually  lined  with  white  metal  or  anti-friction  metal. 

Gunmetal  is  composed  of  copper,  tin,  and  zinc ;  the  amount  of 
each  is  varied  slightly  according  to  the  purpose  for  which  the 
alloy  is  required.  The  proportions  adopted  by  the  Admiralty 
are — copper,  88  parts ;  tin,  10  parts ;  zinc,  2  parts.  By  adding 
more  tin  the  alloy  is  made  harder.  The  component  metals,  when 
the  required  casting  is  of  small  or  medium  size,  are  melted  down 
in  plumbago  pots  or  crucibles ;  when  a  large  casting  is  required 
the  metals  are  melted  down  in  a  furnace. 

Gunmetal  will  safely  bear  a  simple  tensile  stress  of  1J  to  2 
tons  per  square  inch.  The  breaking  stress  is  from  10  to  16  tons, 
with  an  elongation  of  1^  to  10  per  cent.  The  weight  of  a  cubic 
inch  is  -3  Ib.  The  melting  point  is  about  1,800°  R 

Phosphor  Bronze  has  all  the  properties  of  gunmetal,  but  is 
stronger ;  when  hot  it  can  be  forged  and  rolled  into  rods.  It  is 
composed  of  copper,  tin,  and  phosphorus,  the  Admiralty  propor- 
tions being  copper  83,  tin  10,  and  phosphide  of  copper  7.  Phos- 
phor bronze  is  used  for  valve  spindles,  pump-rods  (where  steel 
should  not  be  used  on  account  of  corrosion),  bearings,  and  for 
small  castings  in  cases  where  greater  strength  is  required  than 
can  be  obtained  by  the  use  of  gunmetal. 

Phosphor-bronze  rods  and  forgings  will  safely  bear  a  tensile 
stress  of  4  to  5  tons  per  square  inch,  and  castings  from  2  to 
3  tons.  The  breaking  stress  is  from  10  to  30  tons,  with  an 
elongation  of  10  to  30  per  cent,  on  2  inches.  The  weight  of  a 
cubic  inch  is  '3  Ib. 

Manganese  Bronze,  Stone's  Bronze,  and  Delta  Metal 
have  all  the  advantages  of  and  are  stronger  than  phosphor 
bronze  j  in  fact,  they  can  be  made  stronger  than  mild  steel,  but 
at  the  expense  of  ductility.  All  these  bronzes  contain  a  small 
amount  of  manganese,  iron,  and  other  hardening  materials ;  the 
exact  proportions  are,  however,  a  trade  secret.  These  bronzes 


10  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

are  largely  used  for  pump-rods,  valve  spindles,  ships'  propellers, 
and  in  cases  where  a  strong  non-corrodible  rolled  rod  or  casting 
is  required.  Manganese  bronze,  Stone's  bronze,  and  Delta  metal 
rods  will  safely  bear  a  tensile  stress  of  5  to  7  tons  per  square 
inch,  and  castings  from  3  to  7  tons.  The  breaking  tensile  stress 
of  these  rods  varies  from  29  to  35  tons,  with  an  elongation  of 
10  to  32  per  cent,  on  2  inches. 

Aluminium  Bronze. — An  exceedingly  strong  and  ductile 
bronze  is  made  by  a  mixture  of  90  parts  of  copper  and  10  parts 
of  aluminium.  The  ultimate  breaking  stress  of  this  alloy,  when 
cast,  is  about  30  tons  per  square  inch  in  simple  tension,  with  an 
elongation  of  20  per  cent,  on  2  inches.  When  rolled,  a  tensile 
strength  of  38  tons,  with  an  elongation  of  28 -8  per  cent,  on 
2  inches,  has  been  obtained.  If,  however,  the  alloy  is  subjected 
to  a  temperature  of  about  570°  F.,  the  strength  falls  off  and  the 
alloy  becomes  brittle.  At  a  temperature  of  750°  F.  the  alloy  is 
so  brittle  as  to  be  of  little  use.  The  weight  of  a  cubic  inch  is 
•273  Ib. 

Muntz  Metal  is  more  ductile,  but  not  quite  so  strong  as  the 
bronzes  just  mentioned.  This  metal  consists  of  from  60  to 
62  per  cent,  of  copper  and  from  40  to  38  per  cent,  of  zinc.  It 
is  very  suitable  for  condenser  tubes,  and  for  bolts  which  have  to 
resist  the  corrosive  action  of  sea  water.  The  safe  tensile  stress 
for  Muntz  metal  rods  is  about  5J  tons  per  square  inch,  the 
breaking  stress  about  26  tons,  with  an  elongation  of  45  per  cent. 
on  2  inches. 

Brass  is  inferior  in  strength  to,  and  more  brittle  than,  the 
above-mentioned  alloys.  It  is  composed  of  copper  and  zinc,  with 
a  small  proportion  of  tin.  The  safe  tensile  stress  is  about  1  ton, 
or  less,  per  square  inch. 

White  Metal,  sometimes  called  Babbit  metal,  from  the  name 
of  the  metallurgist  who  first  introduced  it,  is  very  soft,  and  has 
valuable  anti-friction  properties — i.e.,  it  remains  cool  when  sub- 
ject to  rubbing  contact  under  heavy  pressure,  and  for  this  reason 
it  is  largely  employed  for  lining  bearings.  White  metal  is 
composed  principally  of  tin,  with  an  addition  of  antimony  and 
copper.  The  Admiralty  proportions  are  tin  85  to  89,  copper  7  to 
2,  and  antimony  8  to  9.  A  cheaper  form  of  white  metal  is  made 
of  lead  and  a  small  proportion  of  antimony,  but  is  not  considered 
to  be  so  good.  White  metal  is  usually  bought  in  ingots  from 
firms  who  have  made  a  study  of  the  subject.  The  weight  of 
anti-friction  metals  made  from  tin  and  antimony  is  about  '26  Ib. 
per  cubic  inch.  The  weight,  when  made  of  lead  and  antimony, 
is  about  '4  Ib.  per  cubic  inch.  With  regard  to  the  strength, 
Professor  Goodman  has  recently  found  that  the  ultimate 


MATERIALS. 


11 


strength  of  some  white  metal,  consisting  of  lead  90  parts  and 
antimony  10  parts,  was  about  3J  tons  in  tension  and  7-J  tons 
in  compression. 

In  making  castings  of  iron,  steel,  gunmetal,  or  of  any  other 
alloy,  it  is  essential  for  strength  that  sharp  corners  should  be 
avoided,  for  in  cooling  the  crystals  set  in  a  direction  at  right 
angles  to  the  face  of  the  casting,  and  every  sharp  corner  consti- 
tutes a  source  of  danger.  Any  sudden  increase,  too,  in  the 
thickness  of  the  metal  should  be  avoided.  It  has  been  found, 
for  example,  that  a  flat  plate  strengthened  by  ribs  running  into 
a  common  centre  or  small  mass  of  metal,  as  shown  by  Fig.  1,  is 


Fig.  1.  Fig.  2. 

Incorrect  and  correct  methods  of  ribbing  a  flat  plate. 

not  so  strong  as  when  strengthened  in  the  manner  shown  by 
Fig.  2,  where  the  cooling  of  the  metal  can  take  place  more 
evenly.  A  good  radius  should  be  given  to  the  ribs  as  they  meet 
the  central  ring.  Pulleys  and  small  flywheels,  in  which  the 
section  of  the  rim  differs  greatly  from  that  of  the  arms,  usually 
have  the  latter  cast  in  the  form  of  an  S.  This  form  allows  a 
certain  amount  of  give  while  the  metal  is  cooling. 

Testing  Materials.  —  Until  comparatively  recent  years  it 
was  considered  that  if  the  tensile  strength  and  percentage  of 
elongation  of  a  certain  metal,  or  alloy,  was  known,  a  judgment 
could  be  formed  as  to  whether  such  metal  or  alloy  was,  or  was 
not,  suitable  for  a  given  purpose.  It  has,  however,  been  found 
that  of  two  pieces  of  steel  giving  equally  good  results  both  as  to 
strength  and  elongation,  one  may  be  capable  of  withstanding 
shock  very  much  better  than  the  other,  also  that  one  may  be 
able  to  withstand  reversal  of  stress  better  than  the  other.  Tests 
to  ascertain  the  behaviour  of  materials  under  shock  and  reversal 
of  stress  are  now  frequently  made,  but  before  describing  the 
methods  of  making  these  tests  it  may  be  well  to  state  what  takes 
place  when  a  piece  of  metal  is  tested  to  destruction  in  an  ordinary 
testing  machine,  so  that  the  beginner  may  have  clear  ideas  as  to 
terms,  such  as  stress,  strain,  elastic  limit,  yield  point,  reduction 


12  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

of  area,  &c.  When  a  small  rod  of  metal,  or  test  piece,  is  put 
into  a  testing  machine  and  its  ends  pulled  apart,  the  force 
applied  produces  "  stress  "  in  the  metal  and  the  metal  stretches ; 
the  amount  by  which  the  metal  stretches,  or  its  deformation,  is 
called  "  strain."  Up  to  a  certain  point  metal  retains  its  elas- 
ticity, so  that  if  the  load  is  removed  the  metal  returns  to  its 
original  form.  When  the  stress  has  reached  a  certain  point  the 
strain  or  deformation  increases  out  of  proportion  to  the  stress, 
and  the  metal,  provided  it  does  not  fracture  like  cast  iron,  takes 
a  permanent  set;  the  point  at  which  this  occurs  is  called  the 
elastic  limit  or  yield  point.*  Naturally  the  safe  working  stress 
of  the  metal  or  alloy  is  well  below  the  elastic  limit.  If  still 
further  force  is  applied  after  the  metal  has  reached  its  elastic 
limit,  the  rod  becomes  permanently  stretched,  and  its  area 
becomes  reduced ;  finally,  when  a  certain  stress  is  reached  the 
rod  fractures.  This  stress  is  the  ultimate  or  breaking  stress  of 
the  material,  but,  as  the  area  of  the  rod  has  become  reduced,  the 
ultimate  breaking  stress  can  be  stated  in  two  ways — either  as  so 
many  tons  per  square  inch  of  original  section,  or  as  so  many  tons 
per  square  inch  at  point  of  fracture.  The  former  is  the  most 
useful  information  for  practical  engineers,  the  latter  for  scientific 
investigators  into  the  properties  of  materials. 

The  amount  by  which  the  rod  has  stretched  is  called  its 
"  percentage  of  elongation."  Thus,  if  the  working  length  of  the 
test  piece  is  2  inches,  and  it  stretches  J  inch  before  fracture,  its 
elongation  is  25  per  cent.  As  the  stretching  is  greater  at  the 
point  of  fracture  than  in  the  other  parts  of  the  rod,  the  per- 
centage of  elongation  on  a  long  test  piece  is  less  than  on  a  short 
one,  hence  the  length  of  the  test  piece  should  be  given  at  the 
same  time  as  the  percentage  of  elongation  if  the  information  is 
to  be  of  service. 

Testing  machines  have  been  designed  and  are  in  use  in  many 
Technical  Colleges  and  Laboratories,  which,  by  means  of  multi- 
plying levers  and  pencil,  draw  automatically  a  diagram  showing 
the  actual  deformation  that  takes  place  in  the  metal  when 
subjected  to  gradually  increasing  stress.  Such  diagrams  are 
called  stress-strain  diagrams. 

Impact  Tests. — In  order  to  ascertain  whether  a  given  metal 
or  alloy  is  suitable  for  resisting  shock,  an  impact  test  should  be 
made.  A  very  convenient  and  simple  machine  for  this  purpose 
was  designed  a  few  years  ago  by  Mr.  Izod  and  is  made  by  Messrs. 
A  very.  Izod's  impact  testing  machine  consists  of  a  hammer 
suspended  at  the  end  of  a  swinging  rod.  The  sample  piece  of 

*  The  true  yield  point  occurs  slightly  later  than  the  point  of  elastic 
limit,  but  for  ordinary  purposes  they  may  be  looked  upon  as  identical. 


MATERIALS.  13 

steel  or  alloy  to  be  tested  is  of  small  rectangular  section  and  is 
nicked  to  a  uniform  depth  at  the  place  where  fracture  is  desired ; 
the  test  piece  is  placed  in  a  vice  immediately  under  the  hammer, 
the  latter  is  then  swung  back  to  a  given  distance  and  released ; 
in  falling  the  hammer  strikes  the  test  piece  and  fractures  it. 
The  distance  traversed  by  the  hammer  (shown  by  a  dial  and 
pointer),  after  fracturing  the  test  piece,  shows  the  resistance  of 
the  latter.  When  the  steel  or  alloy  under  test  is  brittle  the 
hammer  travels  much  farther  after  fracture  than  when  the 
material  is  suitable  for  withstanding  shocks.  In  this  connection 
it  may  be  mentioned  that  if  mild  steel  is  heated  up  to  about 
1,330°  F.,  and  then  quenched  in  oil,  its  ability  to  withstand  shock 
is  enormously  increased. 

A  simple  testing  machine  for  ascertaining  the  ability  or 
otherwise  of  metals  to  withstand  alternate  bending  has  recently 
been  devised  by  Captain  Sankey.  The  machine,  which  is  made 
by  Messrs.  Casella,  consists  of  a  fixed  and  a  moving  vice,  the 
latter  being  provided  with  a  long  handle.  The  piece  of  metal  to 
be  tested  is  gripped  by  the  fixed  vice  and  bent  by  means  of  the 
moving  vice  and  handle.  The  moving  vice  is  arranged  so  that 
the  pressure  exerted  in  bending  the  test  piece  is  transmitted 
through  springs,  and  by  an  arrangement  of  ratchet,  dial,  and 
pencil,  a  line  or  rather  arc,  indicating  the  pressure  exerted,  is 
drawn  every  time  the  metal  is  bent,  so  that  both  the  number  of 
times  the  metal  bends  before  fracture  and  the  pressure  exerted 
each  time  are  automatically  recorded.  The  results  of  tests  made 
with  this  machine  agree  very  closely  with  tests  made  with  a 
much  more  elaborate  machine  constructed  by  Professor  Arnold. 

Table  i.  gives  a  rough  summary  of  the  safe  and  ultimate 
tensile,  compression,  and  shearing  stresses  of  various  materials, 
with  the  percentage  of  elongation,  also  the  weight  of  a  cubic  inch. 

Some  tensile  and  compression  tests,  made  by  Mr.  Izod,  upon 
four  pieces  of  crucible  steel  are  embodied  in  the  table,  as  they 
show  clearly  how  the  tensile  strength  of  steel  increases  with  an 
increased  percentage  of  carbon,  while  the  elongation  falls  off. 
With  regard  to  the  bronzes  enumerated,  the  higher  tensile 
strengths  are  only  obtained  at  the  expense  of  ductility — i.e.,  as 
the  strength  increases  the  percentage  of  elongation  falls  off. 

It  may  be  noticed  that  while  the  ultimate  shearing  stress  of 
wrought  iron  and  steel  is  less  than  the  ultimate  tensile  stress  ; 
in  the  case  of  cast  iron  the  ultimate  shearing  stress  is  greater 
than  the  ultimate  tensile  stress.  The  figures,  which  are  based 
upon  trials  recently  made,  are  confirmed  by  some  tests*  made  by 

*  Communicated  to  the  Institution  of  Mechanical  Engineers,  in  con- 
nection with  a  paper  on  "  Shear,"  by  Mr.  E.  G.  Izod,  1905. 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


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MATERIALS. 


15 


Professor  Goodman  of  Leeds,  and  given  in  Table  n.  This  table 
is  instructive,  as  showing  the  reduction  in  area  which  takes  place 
when  certain  metals  and  alloys  are  tested  to  destruction,  and 
how  different  some  of  the  results  appear  when  they  are  given  in 
terms  of  "tons  per  square  inch  of  original  section,"  and  when 
given  in  "tons  per  square  inch  at  point  of  fracture." 

TABLE   II. — TESTS  MADE  BY  PROFESSOR  GOODMAN  AT  LEEDS. 


Ultimate 
Tensile 
Stress- 
Original 
Section. 

Ultimate 
Tensile 
Stress  at 
Point 
of  Fracture. 

Reduction 
in 
Area. 

Ultimate 
Shearing 
Stress- 
Original 
Section. 

Tons 
per  sq.  in. 
10-9 

Tons 
per  sq.  in. 
10-9 

Percentage. 
Nil. 

Tons 
per  sq.  in. 
12-9 

Gunmetal  (soft  and  ductile),    . 
,,          (hard  and  brittle),. 
Moderately  hard  steel,    . 
Mild  steel,       .... 
Wrought  iron  (soft), 
,,             (merchant), 
Copper  (annealed),  . 
,,       (hard-drawn), 
Aluminium,      .... 

14-9 
12-4 

48-0 
23-6 
21-7 
22-6 
14-8 
17-2 
8-8 

19-5 
12-8 
49-8 
29-5 
27'4 
24-7 
23-7 
24-1 
12-7 

30-7 
3-0 
7-4 
67'5 
49-8 
24-6 
65-0 
47-0 
45-5 

14-2 
17-4 
34-0 
18-9 
17'4 
24-5 
11-0 
23-3 
5-6 

.The  tensile  strength  of  steel  and  of  most  of  the  bronze  alloys 
can  be  increased  by  cold  rolling — i.e.,  reducing  the  section  of  the 
rod  by  passing  it  through  rolls.  The  increased  tenacity  is,  how- 
ever, gained  at  the  expense  of  ductility.  Copper  and  iron  wire, 
when  drawn  down  by  successive  operations,  is  much  stronger 
than  when  in  the  form  of  rod.  Copper  wire  will  bear  a  stress 
of  about  25  tons  per  square  inch  (as  against  about  14  tons  in 
the  case  of  copper  rods  or  plates)  even  after  annealing ;  before 
annealing  the  wire  has  even  greater  tenacity. 

Fatigue  of  Metals. — It  has  been  found  from  test  pieces  cut 
from  material  which  has  been  subjected  for  some  time  to  reversal 
of  stresses  and  to  shock,  that  its  percentage  of  elongation  has 
fallen  off  from  the  original  figure.  This  falling  off  in  ductility 
and  real  strength  is  sometimes  referred  to  as  the  "  fatigue  of  the 
material."  Wohler  found  out  over  thirty  years  ago  that  frequent 
reversal  of  stress — i.e.,  from  tension  to  compression,  or  vice  versd  ; 
or  the  frequent  application  and  removal  of  stress  of  the  same 
kind — was  sufficient  to  fracture  a  rod,  even  if  the  maximum 
stress  never  exceeded  one  which  was  safe  when  the  load  was  a 
permanent  or  steady  dead  load. 


16  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

Effect    of   Temperature    on    Metals   and   Alloys.  —  The 

strength  of  steel  and  good  wrought  iron  is  not  adversely 
affected  by  temperatures  up  to  500°  F.,  but  above  this  point 
the  tenacity  begins  to  fall  off.  Cast  iron  appears  to  be  some- 
what unreliable  at  temperatures  below  32°  F.,  and  its  strength 
begins  to  fall  off  at  about  200°  F.  Brass  and  gunmetal  are 
unsuited  for  temperatures  above  400°  F.,  and  should  not  be 
used  in  valves  and  steam  fittings  when  the  steam  is  super- 
heated. Special  alloys  suitable  for  fairly  high  temperatures 
can  be  obtained.  The  strength  of  copper  falls  off  at  a  tempera- 
ture of  about  300°  F. 

Young's  Modulus  of  Elasticity. — The  student  will  fre- 
quently find  references  to  Young's  modulus.  This  modulus  is 
the  ratio  that  a  given  stress  per  unit  of  section  bears  to  a  given 
elongation  within  the  elastic  limit  per  unit  of  length.  This  co- 
efficient of  elasticity  varies  with  the  quality  of  the  material ;  for 
good  wrought  iron  it  is  about  12,000,  for  mild  steel  about  13,000, 
and  for  cast  iron  about  7,000,  if  tons  per  square  inch  are  taken. 
Thus,  if  a  force  of  1 5  tons  should  stretch  a  bar  800  inches  long 
and  1  square  inch  area  to  the  extent  of  1  inch,  the  material  still 
retaining  its  elasticity,  then  the  coefficient  of  the  material  would 
be  12,000. 

Factors  of  Safety. — The  factor  of  safety,  or  number  by 
which  the  breaking  strength  of  a  material  should  be  divided  to 
give  its  safe  strength,  depends  entirely  upon  the  conditions 
under  which  the  material  will  be  used.  In  building  con- 
struction where  the  load  may  be  a  permanent  or  dead  one, 
a  factor  of  safety  as  low  as  3  is  sometimes  taken  for  steel 
work.  In  boiler  work  in  this  country  a  factor  of  safety  of 
about  5  is  usually  taken ;  thus,  if  the  breaking  strength  of 
the  steel  plate  is  30  tons,  the  safe  stress  will  be  assumed  to  be 
6  tons.  In  the  case  of  iron  castings  where  internal  stresses  may 
be  set  up  in  cooling,  and  in  which  there  may  be  small  blow- 
holes, the  factor  of  safety  taken  is  usually  not  less  than  8. 
In  deciding  upon  the  factor  of  safety,  the  results  of  Wohler's 
investigations,  previously  referred  to,  must  not  be  overlooked. 
Professor  Unwin,  who  is  an  authority  on  testing  and  on  the 
strength  of  materials,  considers  that  Wohler's  experiments  show 
roughly  that  if  the  safe  stress  of  a  steel  bar  under  a  steady 
permanent  load  is  called  3,  then  the  safe  stress  for  the  same  bar 
under  a  load  which  is  alternately  removed  and  replaced,  will 
be  2 ;  while  if  the  bar  is  subject  alternately  to  tension  and 
compression,  the  safe  stress  will  be  only  1. 

Quality. — The  following  clauses  dealing  with  the  quality  of 
cast  iron,  mild  steel,  and  cast  steel  are  taken  from  actual 


MATERIALS.  17 

specifications    issued    by   engineers   within    the    last    year    or 
two: — 

"  Cast  Iron. — All  cast  iron  to  be  of  good  close-grained  quality, 
free  from  cracks,  flaws,  blowholes,  or  chilled  spots.  All  cast- 
ings under  working  stresses  to  be  of  metal  to  stand  the  following 
tests,  viz. : — A  test  bar  42  inches  long  by  2  inches  by  1  inch  in 
section  to  be  cast  at  the  same  time  and  from  the  same  ladle. 
The  test  bar  when  placed  on  its  edge  between  supports  36  inches 
apart  to  carry  a  load  (gradually  applied)  in  the  centre,  of 
30  cwts.,  and  to  deflect  under  such  load  *2  inch.  The  harder 
metal  for  the  cylinders  and  liners  to  carry  a  load  under  the 
same  conditions  of  40  cwts.,  and  to  deflect  under  such  load  -3  inch. 

"  Mild  Steel.— The  crank-shaft,  piston-rods,  and  connecting-rods 
to  be  of  mild  steel,  having  a  tenacity  of  from  26  to  30  tons 
per  square  inch, -and  with  an  elongation  of  not  less  than  22  per 
cent,  in  10  inches  before  fracture.  If  required  to  do  so  by 
the  engineers,  the  contractors  must  send  them  ready  prepared 
specimens  (of  dimensions  approved  by  them)  cut  from  the 
forgings  for  the  purpose  of  being  tested. 

"Mild  Steel  Boiler  Plates.— All  the  plates  throughout  each 
boiler,  and  all  the  rivets  and  gusset  stays  are  to  be  made  of 
open  hearth  mild  steel.  All  the  plates  are  to  be  placed  with 
their  brands  visible  on  the  outside  of  the  shell,  or  on  the  inside 
of  the  flues,  as  the  case  may  be.  Every  plate  throughout  the 
boiler  is  to  be  tested  at  the  expense  of  the  contractor  under  this 
specification  by  a  longitudinal  and  transverse  strip  taken  from 
each  plate.  Every  plate  to  be  capable  of  standing  a  tensile 
stress  of  somewhere  between  the  limits  of  27  tons  and  of  30  tons 
per  square  inch  of  original  sectional  area,  either  lengthwise  or 
crosswise  of  the  plates,  with  an  extension  of  not  less  than  20  per 
cent,  in  a  length  of  10  inches,  which  is  to  be  the  length  of  the 
operative  part  of  the  test  piece.  In  no  case  is  the  test  piece  to 
have  a  less  sectional  area  than  half  a  square  inch. 

"  Cast  Steel. — All  cast  steel  to  be  as  free  from  blowholes  as 
practicable,  and  to  be  well  annealed.  All  large  blowholes  to 
be  filled  by  electric  welding,  and  castings  so  treated  to  be 
annealed  after  welding.  Test  bars  to  be  cast  from  the  same 
ladle  at  the  same  time  as  the  bulk  of  the  castings,  and  to  give 
a  tensile  strength  of  27  to  30  tons  per  square  inch,  and  an 
elongation  in  3-inch  test  bars  of  not  less  than  20  per  cent, 
before  fracture. 

"Manganese  Bronze. — All  manganese  bronze  used  for  bolts, 
nuts,  studs,  &c.,  to  be  of  high  tensile  strength,  and  test  bars  to 
show  a  tensile  strength  of  25  to  28  tons  per  square  inch,  and  an 
elongation  of  not  less  than  20  per  cent,  before  fracture." 

2 


18 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


Boiler  makers,  in  ordering  steel  plates  from  the  makers, 
usually  specify  that  a  strip  2  inches  broad  and  10  inches  long, 
sheared  from  the  plate  and  heated  to  redness,  then  cooled  in 
water  at  80°  F.,  must  stand  bending  until  the  inside  radius  of 
the  curve  is  one  and  a-half  times  the  thickness  of  the  plate.  The 
plates  must  stand  this  bending  without  showing  any  signs  of 
fracture. 

Table  in.,  giving  the  approximate  cost  of  the  material  with 
which  an  engineer  has  to  deal,  may  prove  useful  to  a  beginner. 


TABLE   III. — APPROXIMATE  COST  OF  MATERIALS. 


Iron  castings,  plain  and  fairly  heavy, 
,  ,             intricate             ,  , 
Pig  iron  (Scotch),    .... 
Wrought  iron,  j 
or              ,  Plates,  angles,  and  bar 
Mild  steel, 
Steel  castings,         .... 

£9      t 
£12      , 
.     £2  15s.  , 

s,       £6     ,, 

£28      , 
£120 
lOd. 
lOd. 
lid. 
lid. 
£70 
£150 
£170 
£17 
£50 

o     £12     pei 
,     £18 
,  £3  10s. 

£S  10s. 

,      £45 
£130 
Is.  6d.  pe 
Is. 
Is. 
Is. 
£115  per 
£200 
£195 
£22 
£200 

'  ton. 

» 

rib. 
i 

ton. 

Gunmetal  castings, 

Phosphor  bronze  rods,    . 
Manganese          ,, 
Stone's                 ,, 
Copper  ingots  for  melting, 
Tin 
Zinc           „                ,, 
Lead       ^    „ 
White  metal 

i 

N.B. — The  wide  margin  given  in  the  prices  of  tin  and  copper  is  due  to 
the  fact  that  the  price  of  these  metals  fluctuates  very  considerably. 


19 


CHAPTER  II. 


BOLTS  AND  NUTS,  STUDS,  SET  SCREWS  AND  RIVETS. 

BOLTS  and  nuts,  studs,  set  screws  and  rivets  are  made  of 
wrought  iron  or  mild  steel;  the  annexed  illustrations  clearly  show 
the  difference  between  each.  Bolts,  studs,  and  set  screws  are 
used  for  bolting  together  two  pieces  of  metal  which  may  require 
subsequently  to  be  taken  apart.  Bolts  and  nuts  are  used  in  all 
cases  where  there  is  room  for  the  bolt  head.  Studs  are  used 
where  there  is  not  room  for  a  bolt  head,  as  shown  by  Fig.  4,  or 
where  it  is  undesirable  to  make  a  hole  right  through  both  pieces 
of  metal  to  be  fastened  together.  The  objection  to  a  stud  is  that 
should  it  break  off,  it  is  difficult  to  extract  the  portion  of  the 
stud  which  is  screwed  firmly  into  the  metal.  Even  if  the  thread 
only  gets  stripped  off,  or  the  stud  becomes  bent,  it  is  a  troublesome 
matter  to  replace  it.  A  set  screw  is  used  in  cases  where  there 
is  not  room  for  a  bolt  head,  and  where  it  is  undesirable  to  have 
a  projecting  stud  when  one  portion  of  the  joint  has  been  removed. 


Fig.  3.— Bolt  and  nub.         Fig.  4.— Stud.         Fig.  5.— Set  screw. 

In  ca$es  where  bolts  and  nuts  are  subject  to  vibration,  there 
is  a  danger  of  the  nuts  working  loose  and  coming  off.  Various 
devices  are  employed  to  minimise  this  danger :  the  oldest  and 
most  common  one  is  to  use  two  nuts  and  to  lock  them  together. 
The  two  nuts  are  screwed  down  hard,  the  inner  nut  is  then 
unscrewed  for  a  fraction  of  a  turn,  the  outer  nut  being  screwed 


20 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


down  at  the  same  time.  The  nuts  are  thus  pressed  one  against 
the  other  and  are  locked ;  two  spanners  are  required  for  the 
operation.  A  split  pin  is  usually  put  through  the  end  of  the 
bolt  to  prevent  any  chance  of  the  nuts  coming  off,  should  they 
come  unlocked. 


Fig.  6. 
Lock  nuts. 


Fig.  7. 
Grover  washer. 


Fig.  8. 
Castle  nut. 


Another  device  recently  introduced  is  the  Grover  washer, 
which  consists  of  a  split  spring  washer,  as  shown  in  Fig.  7.  When 
the  nut  is  screwed  down  it  compresses  the  washer,  and  thus  the 
threads  of  the  nut  are  always  pressed  tightly  against  those  of 
the  bolt.  Grover  washers  of  small  size  are  of  plain  rectangular 
section  and  not  as  shown  by  the  illustration.  In  the  Helicoid 
nut  (Fig.  9)  the  nut  itself  takes  the  form  of  a  spring  and  grips 
the  bolt. 


Fig.  9. —Helicoid  nut. 

Another  device  is  the  Castle  nut  (Fig.  8) ;  this  nut  is  provided 
with  saw  cuts  or  narrow  grooves ;  a  split  pin  is  passed  through 
one  of  these  grooves  and  through  a  hole  in  the  bolt.  If  one  of 
the  saw  cuts  does  not  happen  to  come  opposite  the  hole  in  the 
bolt,  the  nut  is  removed  and  a  few  touches  with  a  file  given  to 
the  underside  until  one  of  the  grooves  comes  into  the  desired 
position. 

Steady  or  Dowel  Pins. — If  two  pieces  of  metal  are  fastened 
together  by  set  screws  and  are  subject  to  lateral  motion,  such 


Fig.  10. — Set  screw  and  steady  pin.  Fig.  11. — Lewis  bolt. 


BOLTS    AND    NUTS,    STUDS,    SET    SCREWS    AND    RIVETS. 


21 


motion  causes  the  sharp  edges  of  the  threads  to  cut  into  the 
adjoining  metal ;  to  overcome  this,  one  or  more  steady  or  dowel 
pins  are  usually  put  in  as  shown  by  Fig.  10.  A  steady  pin  is 
merely  a  plain  round  piece  of  steel  slightly  tapered  and  driven 
into  a  correspondingly  tapered  hole ;  it  effectually  prevents  any 
lateral  motion. 

Fig.  11  shows  a  Lewis  bolt,  such  as  is  sometimes  used  for 
holding  down  machine-tools  or  small  pieces  of  machinery  to 
concrete  foundations.  The  Lewis  bolts  are  put  into  holes  large 
enough  to  receive  the  largest  part  of  the  bolt ;  the  space  round 
the  bolt  is  then  run  in  with  cement  or  molten  lead.  Large 
pieces  of  machinery  usually  have  long  holding  bolts  provided 
with  square  flat  anchor  plates  at  the  lower  end.  In  some  cases 
the  bolt  has  a  solid  head  under  the  anchor  plate,  in  which  case 
the  chase  or  square  hole  containing  the  bolt  and  anchor  plate  is 
not  run  in  with  cement  until  the  bed-plate  has  been  placed  in 


Fig.  12.— Foundation  bolt  and  anchor  plate. 

position.  In  other  cases  there  is  an  oblong  hole  in  the  anchor 
plate  through  which  the  bolt-head,  also  oblong,  is  dropped.  The 
anchor  plate  is  provided  with  stops  which  prevent  the  bolt-head 
making  more  than  half  a  turn — i.e.,  until  the  oblong  bolt-head  is 
at  right  angles  to  the  oblong  hole.  A  bolt  with  a  head  and  plate 
of  this  description  can  be  withdrawn  and  replaced  should  it  be 
necessary  to  do  so.  The  hole  through  the  foundation  is,  of 
course,  left  sufficiently  large  for  the  head  to  pass. 

In  other  cases  hand  holes  are  provided  in  the  foundations  to 
give  access  to  the  lower  ends  of  the  bolts,  and  a  cotter  or  flat 
piece  of  steel,  as  shown  by  Fig.  12,  is  placed  through  a  slot  in 
the  bolt,  and  serves  as  a  head.  If  it  is  desired  to  replace  the 


22 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


bolt,  the  cotter  is  knocked  out  and  the  bolt  drawn  up  through 
the  foundation.  The  end  of  the  bolt  passing  through  the  anchor 
plate  is  square.  This  gives  increased  section  to  compensate  for 
the  slot,  and  prevents  the  bolt  from  turning  while  the  nut  is 
being  tightened  up. 

The  following  table,  giving  the  number  of  threads  per  inch  of 
a  Whit  worth  screw,  and  the  diameter  of  the  bolt  at  the  bottom 
of  the  thread,  with  the  corresponding  area,  may  be  found 
useful : — 

TABLE   IV.— WHITWORTH  THREADS. 


Diameter  of 
Bolt. 

Number  of 
Threads 
per  Inch. 

Diameter  at 
Bottom 
of  Thread. 

Area  at 
Bottom  of 
Thread. 

Thickness  of 
Head. 

Thickness 
of  Nut  = 
Diameter  of 
Bolt. 

Inches. 

Inches. 

Sq.  Inches. 

Inches. 

Inches. 

| 

20 

•186 

•027 

•219 

i 

| 

16 

•295 

•068 

•328 

1 

12 

•392 

•121 

•437 

I 

I 

11 

•508 

•202 

•547 

§ 

10 

•622 

•303 

•656 

I 

1 

8 

•840 

•553 

•875 

1 

1* 

7 

1-067 

•894 

1-092 

n 

1| 

6 

1-286 

J-298 

1-312 

4 

If 

5 

1-493 

1-750 

1-531 

if 

2 

*J 

1-715 

2-309 

1-75 

2 

H 

4 

2-179 

3-724 

2-18 

«i 

3 

3i 

2-634 

5-439 

2-62 

3 

4 

3 

3-573 

10-027 

3-5 

4 

5 

2| 

4-534 

16-146 

4-5 

5 

6 

2| 

5-489 

23-65 

5-25 

6 

The  ordinary  Whitworth  thread  is  rather  coarse,  and  cuts 
into  the  metal  to  a  considerable  extent.  Wrought-iron  pipes 
for  steam,  water,  and  gas  are  screwed  with  a  much  finer  thread, 
which  is  usually  known  as  "gas  thread."  This  thread  is  often 
used  for  parts  of  machinery  where  a  fine  adjustment  is  required, 
and  where  it  is  inadvisable  to  cut  deeply  into  the  metal.  In 
speaking  of  a  ^-inch  gas  thread,  a  thread  suitable  for  a  pipe, 
the  internal  diameter  of  which  is  \  inch,  is  meant.  The  ex- 
ternal diameter,  the  number  of  threads  per  inch,  and  the 
diameter  at  the  bottom  of  the  thread,  are  given  in  Table  v. 

To  ascertain  what  load  a  rod  of  wrought  iron,  say  1  inch  in 
diameter,  screwed  with  Whitworth  thread  will  safely  carry,  it  is 
necessary  to  take  the  area  of  the  bolt  at  the  smallest  part — viz., 
at  the  bottom  of  the  thread.  From  Table  iv.  it  will  be  seen 


BOLTS    AND    NUTS,    STUDS,    SET    SCREWS    AND    RIVETS. 

TABLE   V.— GAS  THREADS. 


23 


Internal 
Diameter  of 
Pipe. 

Number  of 
Threads 
per  Inch. 

Diameter 
Outside. 

Diameter 
at  Bottom  of 
Thread. 

Inches. 

Inches. 

Inches. 

i 

28 

•382 

•336 

19 

•518 

•456 

g 

19 

•656 

•588 

4 

14 

•825 

•734 

| 

14 

•902 

•810 

14 

1-041 

•949 

| 

14 

1-189 

1-097 

1 

11 

1-309 

1-192 

H 

11 

1-492 

1-375 

U 

11 

1-650 

1-523 

if 

11 

1-745 

1  628 

1J 

11 

1-882 

1-766 

11 

2-021 

1-904 

if 

11 

2-116 

2-000 

2 

11 

2-347 

2-231 

that  the  area  of  a  1-inch  bolt  at  the  bottom  of  the  thread  is  only 
•553  of  an  inch.  If,  however,  it  is  decided  to  use  a  gas  thread, 
so  as  not  to  cut  so  deeply  into  the  metal,  and  we  wish  to  know 
the  area  at  the  bottom  of  the  thread,  we  must  refer  to  Table  v. 
From  this  it  will  be  seen  that  the  gas  thread  of  a  J  pipe  is 
1-041  inches  outside,  so  that  a  J  gas  thread  would  be  used.  The 
diameter  at  the  bottom  of  this  thread  is  *949,  as  compared  with 
•840  in  the  case  of  the  Whitworth  thread. 

In  calculating  the  load  a  bolt  wrill  bear,  it  must  not  be  over- 
looked that  a  considerable  stress  may  be  imparted  to  the  bolt  by 
the  act  of  tightening  up  the  nut,  and  for  this  reason  a  consider- 
able margin  must  be  allowed.  It  is  not  good  practice  to  allow  a 
greater  stress  than  1  ton  per  square  inch  on  bolts  under  |-inch 
diameter,  or  1 J  to  2  tons  per  square  inch  upon  bolts  above  this 
size.  Bolts,  the  nuts  of  which  require  to  be  well  tightened  up, 
should  be  not  less  than  |-inch  in  diameter.  With  ordinary  force 
applied  at  the  end  of  a  spanner  it  is  possible  to  break  a  f-mch 
bolt,  and  to  put  an  undue  stress  upon  a  J-inch  bolt.  Calculation 
shows  that,  neglecting  friction,  a  force  of  10  Ibs.  applied  at  the 
end  of  a  spanner  8  inches  long  is  sufficient  to  cause  a  stress  of 
53  tons  per  square  inch  on  a  §-inch  bolt,  and  22  tons  per  square 
inch  on  a  J-inch  bolt.  In  actual  practice,  however,  the  friction 
between  the  nut  and  the  metal  on  which  it  presses  absorbs  some  of 
the  force  applied  to  the  spanner.  The  friction  between  the  threads 
of  the  nut  and  bolt  transmits  a  torsional  stress  to  the  latter. 


24 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


As  the  diameter  of  a  bolt  increases,  the  area  of  metal  increases 
very  rapidly — viz.,  as  the  square  of  the  diameter,  and  the  stress 
per  square  inch  due  to  tightening  up  falls  off.  In  the  case  of  a 
1-inch  bolt  and  a  force  of  20  Ibs.  applied  at  the  end  of  a  spanner, 
10  inches  long,  a  stress  of  about  8  tons  per  square  inch  is  put  on 
the  bolt.  We  see  from  the  table  that  the  area  of  a  1-inch  bolt 
is  '553  inch  at  the  bottom  of  the  thread,  and  if  we  allow  a  stress 
of  about  1J  tons  per  square  inch,  we  find  that  the  bolt  will  carry 
•83  of  a  ton  in  addition  to  the  stress  put  upon  it  by  tightening 
up  the  nut,  or  8 '83  tons  per  square  inch  altogether.  This,  al- 
though a  fairly  high  stress,  is  well  below  the  elastic  limit  of 
wrought  iron,  and  is  only  about  one-third  of  the  breaking  stress 
of  the  metal. 

The  most  suitable  number  of  bolts,  their  diameter,  and  pitch, 
for  circular  flanges  will  be  found  in  the  table  of  pipe  flanges, 
prepared  by  the  Engineering  Standards  Committee,  given  in 
Chapter  v.  It  will  be  noticed  that  bolts  smaller  than  f-inch 
in  diameter  are  not  used,  even  with  the  smallest  flanges. 

Rivets  are  only  used  for  wrought  iron  and  steel  work,  and  in 
cases  where  the  two  pieces  of  metal  will  not  require  to  be  taken 
apart  at  any  future  time.  Rivets,  unless 
of  very  small  size,  are  heated  before 
being  closed ;  the  contraction  of  the  rivet 
while  cooling  draws  the  plates  tightly 
together.  Large  rivets  are  usually 
closed  by  means  of  hydraulic  riveters ; 
such  riveting  machines  are  described 
in  the  chapter  on  hydraulic  machinery. 
The  size  of  rivets  and  the  distance 

they  are  spaced  apart  or  pitched  depends  upon  the  thickness  of 
the  plate  and  the  kind  of  joint  used.  The  following  tables  give 
approximately  the  dimensions  found  in  practice  : — 

TABLE   VI. 


Tigs.    13   and    14.— Rivet 
before  and  after  closing. 


SINGLE-RIVETED  LAP  JOINT. 

Thickness  of 
Plate. 

Diameter  of 
Rivet. 

Pitch  of 
Rivet. 

Lap  of  Plates. 

Inches. 
t 

Inches. 

H 

Inches. 

JP 

Inches. 
2 

2| 

A 
1 

H 

iT7r 
Ji 

1 

2| 
3 
3i 

BOLTS    AND    NUTS,    STUDS,    SET    SCREWS    AND    RIVETS. 


25 


DOUBLE-RIVETED  LAP  JOINT. 

Thickness  of 
Plate. 

Diameter  of 
Rivet. 

Pitch  of 
Rivet. 

Lap  of  Plates. 

Inches. 

Inches. 

Inches. 

Inches. 

4 

H 

24 

3f 

A 

H 

3 

4* 

1 

i 

3 

44 

H 

i 

3| 

4| 

1 

1TV 

8* 

41 

I 

H 

H 

54 

i 

1& 

4 

64 

DOUBLE-RIVETED  BUTT  JOINTS—  DOUBLE  STRAPS. 

Thickness  of 

Diameter  of 

Pitch  of 

Width  of  Butt 

Plate. 

Rivet. 

Rivet. 

Straps. 

Inches. 

Inches. 

Inches. 

Inches. 

i 

H 

34 

8^ 

1 

34 

8| 

I 

1 

34 

9 

1 

i 

4 

10 

1 

H 

*i 

11 

1* 

li 

54 

12 

14 

IA 

5| 

13 

TREBLE-RIVETED  BUTT  JOINTS  —  DOUBLE  STRAPS. 

Thickness  of 
Plate. 

Diameter  of 
Rivet. 

Pitch  of 
Rivet. 

Width  of  Butt 
Straps. 

Inches. 

1 

Inches. 

1 

Inches. 
3| 

Inches. 
12 
134 

18 

14 

If 

6| 

71 

21 
24 

A  lap  joint,  as  its  name  implies,  is  one  in  which  one  plate 
laps  over  the  other.      A  butt  joint  is  one  in  which  the  two 


26  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

edges  of  the  plate  meet;  separate  straps  or  cover  plates  are 
used  on  one  or  both  sides  of  the  plate. 

The  pitch  referred  to  in  the  tables  is  the  distance  from  centre 
to  centre  of  the  rivets  in  the  rows  running  parallel  to  the  joint. 
In  double  riveting  the  diagonal  pitch — i.e.,  the  distance  from 
the  centre  of  the  rivet  in  one  row  to  the  centre  of  the  rivet  in 
the  next  row,  measured  diagonally,  should  be  not  less  than 
•65  P  +  '35  I) ;  where  P  =  pitch  of  rivets,  D  =  diameter  of 
rivets. 

The  thickness  of  conical  rivet  heads  should  not  be  less  than 
three-quarters  of  the  diameter  of  the  rivet.  The  strength  of 
riveted  joints  is  dealt  with  in  the  chapter  on  boilers. 


CHAPTER   III. 
BOILERS. 

BOILERS  may  be  divided  into  two  distinct  classes — viz.,  the  Shell 
and  the  Water-tube.  Boilers  of  the  Cornish,  Lancashire,  loco- 
motive, and  kindred  types  are  shell  boilers — i.e.,  they  consist  of 
a  cylindrical  shell  containing  the  water  and  steam.  The  shell 
contains  combustion  chambers  in  which  the  fuel  is  burnt,  and 
flues  or  tubes  through  which  the  gases  pass ;  the  combustion 
chamber  and  tubes  are  surrounded  by  water. 

In  the  water-tube  class  of  boiler  the  water  is  contained  in, 
and  circulated  through,  a  large  number  of  comparatively  small 
tubes;  the  tubes  are  connected  either  directly  to,  or  through 
headers  with,  drums  containing  steam  and  water.  Amongst 
the  best  known  water-tube  boilers  are  the  Babcock,  Stirling, 
Thorny  croft,  Yarrow  ;  there  are  many  others.  We  will  first 
consider  boilers  of  the  shell  type,  pointing  out  their  good 
features  and  the  dangers  connected  with  their  use. 

Lancashire  Boiler. — Fig.  15  shows  a  Lancashire  boiler  in 
section,  and  Fig.  16,  a  front  view  of  the  same  boiler.  This 
type  of  boiler  has  been  in  use  for  a  great  many  years,  and  has 
many  points  in  its  favour.  It  is  fairly  inexpensive  to  construct, 
and  is  sufficiently  large  to  allow  of  a  man  getting  inside  to 
inspect  and  to  chip  away  any  scale,  should  such  be  formed. 
When  steam  has  been  raised  it  is  easy  to  maintain  it  at  a  constant 
pressure,  even  when  sudden  demands  for  large  quantities  of 
steam  are  made  upon  it,  owing  to  the  large  volume  of  water 
(ready  to  evaporate)  and  of  steam  which  it  contains.  The 
objections  to  this  boiler  are  those  which  apply  to  all  boilers 
of  the  shell  type,  and  are  dealt  with  later.  Apart  from 
these  objections,  all  that  can  be  urged  against  a  boiler  of  the 
Lancashire  type  is  that  it  occupies  a  good  deal  of  space,  and 
a  considerable  time  is  required  to  raise  steam  if  the  boiler  is 
allowed  to  get  cold. 

Fig.  15  represents  a  30  feet  by  8  feet  Lancashire  boiler 
suitable  for  a  pressure  of  160  Ibs.  ;  there  are  a  few  points 
about  it  to  which  attention  should  be  drawn. 

In  the  first  place,  a  true  section  through  the  centre  of  the 
boiler  would  pass  between  the  two  flues,  and  the  outside  of  one 
flue  only  would  be  seen.  In  order  to  show  the  inside  of  the 


28 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


BOILERS. 


29 


flue,  a  section  following  the  dotted  line  A  A,  B  B  (Fig.  16),  is 
given.  This  conventional  way  of  showing  a  Lancashire  boiler 
has  some  drawbacks — for  instance,  the  gusset  stays,  which 
prevent  the  ends  from  bulging  out  under  the  pressure  of 
steam,  appear  to  go  close  up  to  the  flue,  while  in  reality  there 
is  a  space  of  9  inches  or  10  inches  between  the  stay  and  the  flue, 
as  will  be  seen  from  the  end  view.  The  object  of  this  breathing 
space,  as  it  is  called,  is  to  allow  the  end  plates  to  "give"  slightly 
when  the  flues  expand  or  contract. 

It  will  be  noticed  that  while  the  shell  consists  of  six  rings  of 
rather  wide  plates,  the  flue  is  composed  of  a  much  larger  number 


Fig.  16. — Lancashire  boiler,  with  brick  setting. 

of  narrow  rings.  The  reason  for  this  is  as  follows  : — The  fewer 
plates  there  are  forming  the  shell  the  less  riveting  is  required, 
and  a  sounder  and  less  expensive  boiler  is  produced.  As  much 
wider  plates  can  be  obtained  now  than  was  formerly  the  case, 
the  number  of  rings  forming  the  shells  of  Lancashire  and  other 
boilers  has  been  considerably  reduced.  The  width  of  the  furnace 
rings,  when  joined  together  by  "  Adamson  "  rings,  is  determined, 
on  the  other  hand,  by  the  working  pressure  of  the  boiler,  as  the 
strength  of  the  flue  to  resist  collapsing  pressure  is  largely 
dependent  upon  the  number  and  distance  apart  of  these  rings. 


30  MECHANICAL   ENGINEERING    FOR    BEGINNERS. 

In  a  boiler  constructed  for  only  80  Ibs.  working  pressure,  the 
furnace  rings  may  be  about  3  feet  6  inches  wide,  and  the 
Adamson  rings  be  spaced  this  distance  apart  from  one  another ; 
but  for  a  boiler  to  work  at  a  pressure  of  160  Ibs.,  the  furnace 
rings  should  only  be  2  feet  3  inches  wide ;  while  for  a  pressure 
of  200  Ibs.  the  rings  should  be  still  narrower — viz.,  about  2  feet 
1  inch  wide. 

An  enlarged  sectional  view  of  the  Adamson  ring  is  given  in 
the  top  right-hand  corner  of  Fig.  15.  It  will  be  noticed  that  the 
rivet  heads  are  not  exposed  to  the  flames  or  gases,  while  the 
central  plate  enables  the  joint  to  be  caulked. 

The  figure  shows  four  Galloway  cross  tubes  in  dotted  lines 
only,  as  the  modern  tendency  is  to  dispense  with  them.  These 
tubes  give  increased  heating  surface,  and  for  many  years  were 
believed  to  improve  the  circulation  of  water  in  a  boiler,  but  the 
manner  in  which  the  tubes  become  coated  with  scale  has  made 
some  engineers  sceptical  as  to  whether  the  circulation  through 
them  is  as  rapid  as  was  thought.  In  any  case,  as  soon  as  a 
tube  becomes  coated  with  scale,  its  efficiency,  from  the  point  of 
view  of  heating  surface,  rapidly  falls  off.  The  tubes  act  very 
efficiently  as  cross  struts  to  support  the  flue,  but  by  placing 
Adamson  rings  closer  together,  as  already  described,  cross  tubes 
as  struts  may  be  dispensed  with.  Leakage  at  the  flanged  joints 
of  these  tubes  is  also  sometimes  experienced,  and  to  avoid  this 
some  boiler  makers  have  welded  in  the  cross  tubes,  but  this 
operation  is  not  altogether  an  easy  one,  and,  unless  the  welding 
is  very  thoroughly  done,  it  may  lead  to  trouble. 

The  old-fashioned  steam  dome  has  been  discarded  for  many 
years  on  Lancashire  and  Cornish  boilers.  The  reasons  are  two- 
fold; in  the  first  place,  the  dome  did  not  ensure  dry  steam  passing 
to  the  engine,  and  in  the  second  it  weakened  the  boiler  con- 
siderably. With  regard  to  obtaining  dry  steam,  it  has  been 
found  that  if  a  large  quantity  of  steam  is  collected  from  an 
aperture  placed  over  a  small  area  of  water,  the  rush  of  steam 
carries  with  it  small  particles  of  water,  and  this  is  called 
"priming."  An  anti-priming  pipe,  as  shown  by  Fig.  15,  is  now 
generally  fitted ;  this  is  merely  a  pipe  with  closed  ends,  having  a 
large  number  of  slots  or  perforations  in  the  upper  half  of  its 
circumference.  This  pipe  collects  steam  evenly  from  a  fairly 
large  area,  and  is  more  effective  than  the  old-fashioned  steam 
dome. 

The  fitting  shown  at  the  extreme  left  of  the  boiler  consists  of 
one  spring  loaded  and  one  dead  weight  safety  valve  mounted  on 
a  common  seating.  The  spring  safety  valve  is  usually  provided 
with  a  lever,  by  moving  which  (by  a  chain  or  otherwise)  the 


BOILERS.  31 

valve  may  be  raised  and  steam  blown  off  through  a  pipe  fitted  to 
it.  The  dead  weight  safety  valve  is  usually  loaded  to  blow  off 
at  5  Ibs.  greater  pressure  than  the  spring  valve,  the  steam 
escaping  to  the  boiler-house. 

The  fitting  shown  to  the  right  of  the  stop  valve  and  anti- 
priming  pipe  is  a  high  pressure  and  low  water  alarm  safety  valve. 
If  the  level  of  the  water  falls  too  low  the  float  sinks  and  opens 
a  small  valve  which  may  communicate  with  a  whistle;  in  the 
figure  the  float  is  apparently  almost  touching  the  flue,  because 
the  figure  does  not  show  a  true  section ;  in  reality  the  float 
can  descend  for  a  short  distance  between  the  two  flues. 
Surrounding  the  small  valve,  actuated  by  the  float,  there  is 
another  valve  which  acts  as  an  ordinary  safety  valve  should  the 
steam  pressure  rise  too  high. 

The  manhole  shown  to  the  right  of  the  low  water  alarm  is  to 
enable  a  man  or  boy  to  get  inside  the  boiler  for  examination  or 
for  cleaning  :  a  strengthening  ring  is  riveted  round  the  shell,  just 
below  the  manhole. 

The  elbow  at  the  bottom  of  the  boiler  is  for  blowing  out  any 
sediment  which  may  have  accumulated.  The  elbow  is  usually 
provided  with  a  cock ;  this  cock  is  one  of  the  most  troublesome 
fittings  about  a  boiler,  as  it  gets  cut  by  the  outrushing  steam, 
water,  and  dirt.  Some  users  employ  a  cock  next  the  boiler  with 
a  valve  beyond  it ;  by  keeping  the  latter  closed  until  the  cock  is 
fully  opened,  the  cutting  action  which  is  supposed  to  occur 
while  the  cock  is  being  opened  is  thus  minimised.  Any  slight 
leakage  of  the  outer  valve  is  immaterial. 

The  smaller  fittings,  such  as  the  steam  pressure  gauge  for 
showing  the  pressure  of  steam,  and  the  water  gauge  fittings  for 
showing  the  level  of  the  water  in  the  boiler,  are  not  shown. 
The  latter  are  usually  provided  with  thick  glass  shields  or 
protectors,  or  the  glass  tubes  are  partly  surrounded  by  a  brass 
casing,  so  that  in  the  event  of  the  glass  breaking,  the  pieces 
may  not  strike  the  stoker  in  the  face. 

The  following  table,  giving  the  sizes  and  approximate  evapora- 
tion of  some  Lancashire  boilers  of  the  usual  sizes,  may  be  found 
useful : — 


32  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

TABLE  VII. — LANCASHIRE  BOILER. 


Size  of  Boiler. 

Diameter 
of  Flue. 

Grate 
Area. 

Heating 
Surface. 

Approximate 
Evaporation. 

Ft.      Ins.         Ft.     Ins. 

Ft.    Ins. 

Sq.  Ft. 

Sq.  Ft. 

Lbs.  per  hour. 

20       0      x      6       6            27 

20 

507 

4,250 

22      0     x      6      6           27 

22 

563 

4,700 

24      0     x     6      6           2      7 

24 

620 

5,200 

24      0     x      7       0 

2      9 

25 

670 

5,600 

26      0     x     7      0 

2       9 

27 

730 

6,000 

28      0x7      0 

2       9 

30 

790 

6,500 

30      0     x     7      0 

2      9 

33 

850 

7,000 

30      0     x     7      6 

3      0 

36 

890 

7,500 

28      0     x     8       0 

3      2 

36 

920 

7,500 

30      0     x     8      0 

3      2 

38 

960 

8,000 

30      0     x     8      6 

3      4 

40 

1,010 

8,500 

Note. — The  evaporation  given  for  each  boiler  is  "from  and  at  212°  F.,"* 
and  is  based  on  the  assumption  that  between  20  and  25  Ibs.  of  coal  are 
burnt  per  square  foot  of  grate  per  hour.  With  a  mechanical  stoker  more 
coal  than  this  can  be  burnt  and  greater  evaporation  obtained  ;  with  hand 
firing,  unless  the  fireman  is  very  capable,  less  fuel  will  probably  be  burnt 
and  a  smaller  evaporation  obtained. 

Cornish  Boiler. — This  boiler  is  similar  to  the  Lancashire, 
but  is  smaller,  and  has  only  one  internal  flue  instead  of  two ;  a 
front  view  of  this  boiler  is  shown  by  Fig.  17.  A  Cornish  boiler 

I 


Fig.  17. — Cornish  boiler. 

is  suitable  in  cases  where  an  evaporation  of  1,000  to  4,000  Ibs. 
of  water  is  required  per  hour.  The  following  is  a  list  of  the  sizes 
usually  made,  with  the  approximate  evaporation  which  may  be 
expected : — 

*  The  explanation  of  this  expression  is  given  later. 


BOILERS. 

TABLE  VIII. — CORNISH  BOILER. 


33 


Size  of  Boiler. 

Diameter 
of  Flue. 

Heating 
Surface. 

Approximate 
Evaporation. 

Ft.    Ins.           Ft.   Ins. 

Ft.    Ins. 

Sq.  Ft. 

Lbs.  per  hour. 

14      0x5      0 

2       8 

220 

1,250  to  1,700 

16      0     x     5      0 

2       8 

250 

1,400        1,900 

18      0      x      5      0 

2       8 

280 

1,600        2,100 

20      0      x     5       0 

2       8 

310 

1,750       2,300 

18      0x6      0 

3      2 

340 

1,950        2,600 

20      0     x      6       0 

3      2 

380 

2,200       2,900 

22      0     x      6       0 

3      2 

400 

2,400       3,200 

24      0     x      6       0 

3      2 

440 

2,600        3,500 

2(5      0x6       6 

3      3 

530 

3,000  .     4,000 

In  the  majority  of  places  where  Cornish  boilers  are  installed, 
the  stoker  has  duties  other  than  those  connected  with  the  boiler ; 
under  these  circumstances  a  boiler  which  is  somewhat  large  for 
the  output  required  is  usually  preferred,  so  that  the  stoking  may 
be  done  intermittently,  without  causing  any  serious  fluctuations 
in  the  steam  pressure. 

A  Galloway  boiler  is  a  modification  of  the  Lancashire ;  the 
two  tubes  are  merged  into  one  for  a  great  part  of  their  length,  and 
a  large  number  of  cross  tubes  are  fitted.  It  is  claimed  that  a 
Galloway  boiler  will  evaporate  more  water  than  a  Lancashire 
boiler  of  the  same  external  dimensions,  but,  the  form  of  the  flue 
not  being  truly  cylindrical,  it  is  not  considered  so  suitable  for 
withstanding  high  pressures  as  the  flue  of  a  Lancashire  boiler. 

The  Economic  boiler  (manufactured  by  Davey,  Paxman  & 
Co.)  is  similar  to  a  Cornish  boiler,  but  is  provided  with  a  number 
of  small  tubes  running  parallel  to  the  main  flue,  through  which 
the  hot  gases  return.  This  boiler  occupies  less  space  than  the 
Cornish,  but  more  time  requires  to  be  spent  upon  it  in  cleaning 
the  tubes  and  keeping  them  tight.  The  boilers  just  described 
are  set  in  brickwork,  flues  being  formed  in  it  to  bring  the  hot 
gases  back  alongside  the  outside  of  the  shell ;  these  gases  then 
return  underneath  the  boiler  to  the  chimney. 

Fig.  18  shows  a  Locomotive  boiler.  This  type  of  boiler  is 
employed  in  cases  where  a  large  amount  of  steam  is  required 
from  a  comparatively  small  boiler,  where  portability  is  required, 
and  where  brick  setting  is  not  admissible.  It  is  used,  as  its 
name  implies,  in  locomotives,  also  for  portable  engines,  traction 
engines,  road  rollers,  &c. ;  it  is  occasionally  used  in  steam  in- 
stallations of  a  temporary  nature,  and  on  some  torpedo  boats. 
When  used  on  a  locomotive  the  boiler  is  capable  of  generating 

3 


34 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


OF    THE 

UNIVERSITY 

OF 


BOILERS. 


35 


36 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


more  steam  than  when  used  in  a  stationary  position ;  this  is  due 
partly  to  the  powerful  draught  caused  by  the  blast  of  exhaust 
steam,  and  partly  to  the  vibration  which  is  believed  to  free 
the  bubbles  of  steam  from  the  tubes. 

In  an  English-built  locomotive  the  fire-box  is  usually  made  of 
copper.  The  tubes,  which  vary  from  If  to  2  inches  diameter, 
are  made  of  steel,  iron,  or  brass.  The  locomotive  boiler  is 
somewhat  expensive  to  construct,  owing  to  the  large  amount  of 
staying  which  the  flat  surfaces  of  the  fire-box  require;  these 
stays  are  frequently  a  source  of  trouble,  owing  to  leakage  and 
breakages.  A  recent  innovation  on  the  Lancashire  and  Yorkshire 
Railway  is  a  mild  steel  corrugated  fire-box  which  is  said  not  to 


Fig.  19a.— "  Blake  "  boiler. 

require  stays.  The  fire-boxes  of  ordinary  portable  (loco,  type) 
boilers  are  usually  made  of  the  best  Lowmoor  wrought  iron,  or 
of  mild  steel. 

Fig.  19  shows  a  Marine  boiler  of  the  single-ended  type, 
which  requires  no  brick  setting;  it  is  made  of  large  diameter 
and  of  small  length,  so  as  to  utilise  to  the  greatest  advantage  the 
shape  of  the  ship's  hull.  Marine  boilers  of  the  double-ended 
type  consist  practically  of  two  boilers  as  shown,  placed  back  to 
back,  but  with  one  combustion  chamber  common  to  both. 


BOILERS.  37 

A  Dry  Back  boiler  is  similar  to  the  single-ended  marine, 
but  the  back  of  the  combustion  chamber  is  formed  of  brick- 
work. 

The  marine  boiler  is  fairly  satisfactory  in  use ;  its  disadvantages 
are— the  great  weight  of  the  boiler  when  full  of  water,  and  the 
length  of  time  required  to  get  up  steam.  The  stays  in  the 
combustion  chamber  sometimes  give  trouble,  and  the  tubes 
require  to  be  expanded  occasionally. 

Vertical  Boilers.  —  A  large  variety  of  vertical  boilers 
requiring  no  brickwork  are  made,  and  are  suitable  for  cases 
where  a  small  quantity  of  steam  only  is  required.  The  boiler 
consists  of  a  vertical  shell,  having  an  internal  fire-box ;  there 
are  either  cross  tubes  upon  which  the  flames  impinge,  or  a 
number  of  small  tubes  through  which  the  gases  pass  on  their 
way  to  the  chimney.  These  boilers  are  compact  and  handy, 
but  are  not  economical  in  fuel,  as  a  large  percentage  of  the 
heat  generated  passes  away  through  the  chimney. 

One  of  the  most  economical  boilers  of  this  type  is  the  Blake 
(Fig.  19a),  made  by  The  Blake  Engineering  Co.,  Ltd.,  Darlington. 
This  boiler  has  no  flat  surfaces  and  no  stays. 

Dangers  of  the  Shell  Boiler. — Unless  boilers  of  the  shell 
type  are  examined  periodically  their  use  is  attended  with  serious 
risk  of  explosion,  due  to  the  plates  becoming  weakened,  either  by 
grooving,  pitting,  or  corrosion.  Owners  of  such  boilers  usually 
insure  them  against  explosion,  and  the  Boiler  Insurance  Co. 
periodically  sends  specially  qualified  men  to  examine  and  report 
as  to  the  condition  of  the  boilers.  Grooving  consists  of  the 
formation  of  grooves  in  the  plates  of  a  boiler,  usually  near  a 
joint;  the  grooves  are  sometimes  deep  and  narrow,  and  sometimes 
wide  and  shallow.  They  are  thought  to  be  caused  by  undue 
stresses  coming  upon  a  small  area  of  the  boiler  plates  through 
unequal  expansion  and  contraction.  When  the  fire  is  first 
lighted  in  a  Cornish  or  Lancashire  boiler  the  temperature  of  the 
flues  is  raised,  the  outer  shell  remaining  cool,  and  the  expansion 
of  the  flue,  which  in  a  30-feet  boiler  may  be  as  much  as  J  inch, 
tends  to  force  out  the  end  plates.  The  end  plates,  except  in  the 
Thomson  boiler,  referred  to  later,  are  held  in  by  gusset  stays, 
and  thus,  if  the  plates  are  thick  and  unyielding,  undue  stresses 
may  come  upon  portions  of  them. 

It  was  found  some  years  ago  by  Mr.  Andrews,  who  made  a 
study  of  the  corrosion  of  metals,  that  when  two  pieces  of  steel, 
one  of  which  had  been  strained  to  breaking  point,  and  the  other 
subjected  to  very  slight  stress,  were  placed  in  a  salt  solution, 
they  formed  a  galvanic  couple,  and  that  the  galvanic  action,  thus 
set  up,  greatly  increased  the  corrosion. 


38 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


Pitting,  or  the  formation  of  groups  of  small  holes  in  a  boiler 
plate,  is  usually  due,  in  the  first  place,  to  the  presence  of  some 
acid,  or  impurity  in  the  feed  water,  which  attacks  the  plates. 
Wasting  is  the  gradual  decrease  in  thickness  of  the  plates,  due 
to  corrosion. 


To  avoid  the  stresses  set  up  by  the  expansion  of  the  flues,  a 
portion  of  the  flue  is  sometimes  made  corrugated,  as  shown  in 
Fig.  19.  It  will  be  noticed  in  the  illustration  of  the  Lancashire 
boiler  that  the  end  plates  are  stayed,  or  prevented  from  being 


BOILERS.  39 

forced  outwards  by  the  pressure  of  the  steam,  by  gusset  stays. 
The  latest  development  in  Cornish  and  Lancashire  boilers  in  this 
country  consists  of  dishing  the  ends  of  the  boiler  outwards  in 
the  form  of  a  saucer ;  these  ends  being  partly  spherical,  are  said 
not  to  require  stays,  and  thus  one  of  the  sources  of  undue  stress 
in  a  boiler  is  removed.  These  dished  boiler  ends  are,  however, 
stiff,  and  some  engineers  consider  that  corrugated  flues  should  be 
used  when  dished  ends  are  employed.  The  dished  boiler  ends 
have  recently  been  introduced  by  Messrs.  Thomson  &  Co., 
Wol  verham  pton. 

The  plates  of  shell  boilers  are  almost  invariably  made  of  mild 
steel.  The  rivet  holes  are  all  drilled,  and  the  rivets  are  of  mild 
steel. 

WATER-TUBE  TYPE. 

We  will  now  consider  boilers  of  the  water-tube  type.  Fig.  20 
shows  the  Babcock  boiler.  This  boiler  consists  of  a  large 
number  of  tubes,  each  about  4  inches  external  diameter,  inclined 
at  an  angle  and  expanded  at  each  end  into  vertical  rectanglar 
tubes  or  headers ;  these  headers  communicate  with  a  drum  which 
is  half  full  of  water,  the  remainder  of  the  drum  forming  a  space 
for  steam.  The  water  descends  by  the  back  headers,  rises 
through  the  inclined  tubes,  and  passes  up  the  front  headers,  thus 
maintaining  a  very  good  circulation.  The  furnace  is  placed 
under  the  front  end  of  the  tubes ;  the  gases  are  deflected  by  fire- 
bricks, so  that  they  pass  completely  over  and  under  the  whole 
length  of  the  tubes,  the  gases  striking  them  at  right  angles.  The 
boiler,  with  the  exception  of  the  drum,  is  surrounded  by  brick- 
work, but  it  is  slung  from  joists  carried  on  columns,  so  that  it 
is  free  to  expand  and  contract. 

The  principal  advantages  of  the  Babcock  boiler  are  as  follows : — 
Freedom  from  risk  of  explosion. — The  steam  and  water  drum  is 
of  small  diameter,  and  has  no  flat  stayed  surfaces ;  it  is  not 
exposed  to  the  fierce  heat  of  the  furnace,  and  is  not  subject  to 
severe  stresses  owing  to  unequal  contraction  and  expansion. 
The  drum  can  therefore  be  made  exceedingly  strong  for  the 
pressure  it  has  to  sustain.  The  tubes,  which  are  of  comparatively 
small  size,  are  capable  of  withstanding  extremely  high  pressures ; 
even  in  the  rare  event  of  a  tube  failing,  the  result  is  not  very 
serious ;  the  tube  may  be  rent,  but  unless  the  fire  door  should 
happen  to  be  open  at  the  time  (in  which  case  the  stoker  might 
be  scalded),  no  harm  is  done,  and  the  tube  can  be  renewed  at  a 
very  small  expense.  Compared  with  the  destruction  brought 
about  by  an  explosion  of  a  boiler  of  the  shell  type,  the  result  of 
a  burst  tube  is  insignificant. 


40  MECHANICAL    ENGINEERING   FOR    BEGINNERS. 

Ability  to  raise  steam  quickly. — The  water  in  a  water-tube 
boiler  is  very  much  subdivided,  and  as  the  circulation  is 
extremely  good,  steam  can  be  raised  very  quickly ;  this  is  an 
important  point  in  electric  generating  stations  in  towns  where  a 
fog  may  come  on  suddenly. 

The  boiler  occupies  a  small  space ;  the  heating  surface  is  at 
right  angles  to  the  path  of  the  gases,  and  is  thus  in  the  best 
position  for  extracting  the  heat  from  them.  The  thickness  of 
the  metal  through  which  the  heat  has  to  be  transmitted  is  much 
less  than  in  a  boiler  of  the  shell  type.  For  marine  work  the 
Babcock  boiler  is  slightly  modified ;  smaller  tubes  are  used  and 
a  metal  casing,  lined  with  specially  light  fire-bricks,  takes  the 
place  of  brickwork  setting.  Boilers  of  the  water-tube  type  have 
a  great  advantage  over  shell  boilers  for  war  vessels,  owing  to 
their  ability  to  raise  steam  quickly.  Their  light  weight,  due 
to  the  small  amount  of  water  held,  is  a  point  greatly  in  their 
favour.  The  disadvantages  of  this  type  of  boiler  are  dealt  with 
further  on. 

The  overall  length  of  a  Babcock  boiler  is  usually  about  23  feet, 
irrespective  of  its  evaporative  capacity,  but  shorter  boilers  are 
made  if  required.  The  width  may  vary  from  6  feet  to  12  feet. 
The  evaporation  ranges  from  3,000  to  20,000  Ibs.  per  hour  per 
boiler. 

Niclausse  Boiler. — This  boiler  is  somewhat  similar  to  the 
Babcock,  but  the  tubes  are  connected  to  a  header  at  one  end 
only,  by  means  of  coned  joints,  the  other  end  of  the  tube  being 
closed.  The  circulation  is  obtained  by  placing  one  tube  inside 
another,  and  dividing  the  header  by  a  diaphragm ;  the  water 
passes  down  the  front  portion  of  the  header,  flows  through  the 
inner  tube,  returns  through  the  outer  tube,  and  passes  up 
through  the  back  portion  of  the  header.  The  advantage 
claimed  for  this  boiler  is  that  the  tubes  can  quickly  be 
removed  for  inspection  and  be  replaced,  while  with  most  other 
water-tube  boilers  a  tube  can  only  be  removed  by  cutting  it  out, 
and  inserting  a  new  one.  The  water  in  the  Niclausse  boiler 
cannot  periodically  be  blown  off,  as  the  lower  ends  of  the 
incline'd  tubes  are  closed  ;  this  may  be  considered  a  disadvantage. 
This  boiler  has  had  considerable  success  in  France,  but  has 
made  little  headway  in  this  country. 

The  Belleville  boiler  has  no  headers  back  or  front.  Each 
tube,  which  is  of  a  zig-zag  form,  receives  its  water  at  the  lower 
end,  and  delivers  the  steam  and  water  in  a  state  of  froth  or  foam 
into  a  reservoir  placed  high  up  at  the  front  end  of  the  boiler, 
and  at  right  angles  to  the  tubes.  This  boiler  requires  very 
careful  stoking. 


BOILERS.  41 

Stirling,  Thornycroft,  and  Yarrow  boilers  are  all  variants 
of  the  same  type  of  boiler.  They  have  upper  and  lower  drums 
connected  by  small  tubes,  either  inclined  or  bent.  These  boilers 
have  no  headers  such  as  are  used  in  the  Babcock  and  Niclausse 
boilers.  The  tubes  of  the  Yarrow  boiler  are  straight,  and  so 
give  facilities  for  cleaning;  those  in  the  Thornycroft  and  Stirling 
boilers  are  bent.  All  these  boilers  have  proved  successful ;  the 
Thornycroft  and  Yarrow  are  used  chiefly  for  marine  work,  where 
distilled  water  is  used.  There  are  many  other  water-tube  boilers, 
such  as  the  Hornsby,  Climax,  and  others,  but  they  vary  chiefly 
in  design,  and  not  in  principle. 

The  disadvantage  of  the  earlier  water-tube  boilers  was  their 
lack  of  steam  and  water  space,  so  that  if  an  extra  demand  for 
steam  was  made  upon  them,  there  was  a  tendency  to  prime,  or, 
in  other  words,  for  small  particles  of  water  to  be  carried  off  with 
the  steam.  They  also  required  more  attention  from  the  stoker 
to  keep  the  steam  pressure  and  water  level  constant  than  a  boiler 
of  the  Lancashire  or  Scotch  marine  type.  In  the  Babcock  boiler 
these  disadvantages  have  been  overcome  by  using  a  large  steam 
drum ;  in  the  larger  boilers  of  this  make  two  drums  are  provided, 
placed  side  by  side. 

Strength  of  Boilers. — Some  simple  calculations  as  to  the 
strength  of  boilers  which  a  young  engineer  may  be  called 
upon  to  make  will  now  be  explained. 

The  force  exerted  by  the  steam  acting  at  right  angles  to  the 
surface  of  the  water,  and  tending  to  burst  or  tear  a  boiler 
longitudinally,  is  found  by  multiplying  the  internal  diameter 
of  the  boiler  in  inches  by  the  steam  pressure  in  pounds  per 
square  inch.  To  resist  this  bursting  force  there  is  the  thick- 
ness of  the  metal  plate  on  each  side  of  the  boiler,  so  that, 
ignoring  for  the  moment  the  question  of  riveting,  the  stress  per 
square  inch  in  the  metal  plate  forming  the  shell  of  the  boiler  can 
be  found  thus — 

D  x  P 

TT~T  =  stress; 

where  D  =  diameter  of  the  boiler  in  inches. 
P  =  pressure  of  the  steam  in  Ibs. 
T  =  thickness  of  the  plate  in  inches,  or  parts  of  an  inch. 

Example. — What  is  the  stress  in  the  metal  plates  (undrilled  portion)  of 
a  boiler  8  feet  in  diameter  working  at  150  Ibs.  pressure,  the  plates  being 
|  inch,  or  '75  inch  thick  ?  The  calculation  is 

96X150  =  9,600  Ibs., 


2  x  -75 
or  about  4 '3  tons  per  square  inch. 


42  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

The  calculation  does  not,  however,  take  into  account  the  fact 
that  the  plate  is  weakened  by  the  holes  which  have  been  drilled 
in  it  for  the  riveting.  In  order  to  find  the  stress  upon  the 
weakest  portion  of  the  boiler  shell,  another  factor  is  introduced, 
and  the  formula  reads — 

D  x  P 


2T  x"K 


where  K  =  the  percentage  of  strength  of  the  joint,  the  method 
of  finding  which  will  be  given  later. 

Example.  —  What  is  the  stress  in  the  weakest  part  of  the  plate  of  a  boiler 
8  feet  in  diameter  working  at  150  Ibs.  pressure,  the  plates  being  |  inch 
thick,  and  the  percentage  of  strength  of  the  treble-riveted  butt  joint 
79  per  cent?  The  calculation  is 

96  x  150 

2x.75x-79  =  1 
or  about  5*4  tons  per  square  inch. 

It  is  considered  that  the  tensile  stress  upon  mild  steel  plates 
of  Cornish  or  Lancashire  boilers  having  butt  joints  at  the  longi- 
tudinal seams  should  not  exceed  12,500  Ibs.  per  square  inch,  so 
that  the  thickness  of  the  boiler  shell  in  the  example  just  given 
is  about  right.  The  figure,  12,500,  is  arrived  at  thus:  —  The 
tensile  strength  of  the  boiler  plate  is  probably  about  28  tons, 
and,  allowing  a  factor  of  safety  of  about  5,  we  get  the  figure  in 
question. 

To  find  the  thickness  of  shell  for  any  given  steam  pressure, 
assuming  we  decide  to  allow  a  stress  of  12,500  Ibs.  per  square 
inch,  the  formula  would  be  transposed  thus  — 

D  x  P 

-  -  -  =.  =  thickness. 

2  x   12,500  x  K 

Example.  —  How  thick  should  be  the  shell  of  a  boiler  8  feet  in  diameter 
for  a  working  pressure  of  200  Ibs.  ? 


so  that  a  plate  1  inch  thick  would  be  used.  It  must  be  noted  that  a 
strength  of  79  per  cent,  is  only  obtained  with  a  treble-riveted  butt  joint  ; 
with  a  double-riveted  butt  joint  the  strength  would  only  be  about  '75 
that  of  the  undrilled  plate. 

Apart  from  the  effort  of  the  steam  to  burst  the  boiler  longi- 
tudinally, there  is  the  efibrt  to  extend  the  boiler  lengthwise, 
or  to  pull  it  into  two  parts,  owing  to  the  pressure  of  steam  on 
the  ends.  The  stress  on  the  plates  due  to  this  effort  is,  how- 
ever, only  half  the  stress  due  to  the  pressure  acting  in  the 


BOILERS.  43 

direction  previously  considered,  so  that  if  the  plates  are  strong 
enough  to  withstand  the  radial  pressure,  they  are  of  ample 
strength  for  the  tension  due  to  the  pressure  on  the  ends  of 
the  boiler. 

Dealing  now  with  the  strength  of  the  boiler  ends,  it  may  be  said 
that  the  thickness  of  flat-stayed  surfaces  has  been  arrived  at 
from  the  results  of  experience,  and  that  empirical  formulae  have 
been  arranged  to  suit.  The  formula  given  by  Mr.  Hillier,  chief 
engineer  of  the  National  Boiler  Insurance  Co.,  is  simple  and 
corresponds  very  nearly  with  actual  practice ;  it  is  as  follows  : — 

C  x  T2 

p=^> 

where  P  =  suitable  working  pressure  in  Ibs.  per  square  inch. 
C  =  a  constant  given  below. 
T  =  thickness  of  plate  in  sixteenths  of  an  inch. 
S  =  area  supported  by  one  stay. 

In  Cornish  or  Lancashire  boilers  S  =  the  area  of  the  largest 
circle  which  can  be  got  between  the  gusset  stays,  and  C  =  220 
approximately. 

Example. — In  the  Lancashire  boiler  illustrated,  the  end  plates  are 
|£  inch  thick,  and  the  largest  circle  which  can  be  got  between  the  gusset 
stays  is  16  inches  (area  201).  What  may  the  working  pressure  of  the  boiler 
be  ?  The  calculation  is 

220  x  132       220  x  169 
or =  18o. 

The  answer  is  185  Ibs.  per  square  inch. 

If,  instead  of  being  flat,  the  ends  are  dished  outwards,  we  can 
find  the  stress  per  square  inch  in  the  plate  by  the  following 
formula : — 

R  x  P 

~2 ~;  =  stress ; 

where  R  =  radius  in  inches  of  the  dished  end. 

P  =  pressure  of  the  steam  in  Ibs.  per  square  inch. 
T  =  thickness  of  the  plate  in  inches. 

It  is  not  advisable  to  allow  so  great  a  tensile  stress  in  plates 
which  have  been  dished  when  red  hot,  as  in  plates  which  have 
merely  been  rolled  to  form  the  shell,  and  a  stress  of  6,000  to 
7,000  Ibs.  per  square  inch  is  considered  about  right.  If  we 
decide  to  allow  a  stress  of  6,000  Ibs.  per  square  inch,  and  wish 
to  know  the  thickness  of  a  dished  end,  the  above  formula  is 
transposed  thus — 

R  x  P     _ 
6,000  x  2 


44  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

Example. — How  thick  should  be  the  end  plate  of  a  boiler,  8  feet  in 
diameter,  if  the  plate  is  dished  out  at  a  radius  of  4  feet,  allowing  a  stress 
of  6,000  Ibs.,  the  boiler  pressure  being  185  Ibs.  ? 

48  x  185 

=  '74  thick. 


6,000  x  2 

If  the  end  is  flatter  and  dished  out  to  a  radius  of  6  feet,  the  calculation 
will  be  — 

72  x  185 


- 

6,000  x  2 


=  M  thick. 


If  the  end  is  still  flatter  and  is  dished  out  to  a  radius  of  8  feet,  viz.,  the 
same  as  the  diameter  of  the  boiler,  the  calculation  will  be  — 

96  x  185  =  1.48   h.k 
6,000  x  2 

The  reason  for  multiplying  the  pressure  by  the  radius,  when 
dealing  with  dished  ends,  instead  of  multiplying  the  pressure  by 
the  diameter,  as  when  dealing  with  cylinders,  is  because  a  sphere 
is  twice  as  strong  as  a  cylinder,  and  the  radius  is,  of  course,  only 
half  the  diameter  of  a  circle. 

Strength  of  the  Flues.  —  The  pressure  of  steam  on  the  flue 
tends  to  make  it  collapse  ;  if  the  flue  were  truly  cylindrical,  and 

D  x  P 
the  metal  had  no  tendency  to  buckle,  the  formula  -~  -  m 

would  hold  good,  the  metal  being  in  compression  ;  but,  as  a  fact, 
the  flues  have  a  tendency  to  buckle,  and  the  right  thicknesses 
and  distance  apart  of  the  Adamson  rings  have  been  found  by 
experience. 

Fairbairn's  rule,  which  is  sometimes  quoted,  was  based  on 
experiments  made  many  years  ago  with  plain  tubes,  and  the  rule 
does  not  hold  good  for  flues  having  Adamson  rings. 

The  Board  of  Trade  formula,  which  is  somewhat  elaborate,  is 
given  at  the  foot,  but  the  following  table  will  probably  be  of 
greater  service  to  the  beginner.  The  table  gives  the  distance 
which  the  Adamson  rings  should  be  placed  apart  from  one 
another  for  various  steam  pressures,  and  the  stress  per  square 
inch  which  may  be  allowed  upon  the  metal  forming  the  flue. 


9,900  x  T  /       L  +  12 
Working  pressure  =     —^-   (5  -  - 

where  T  =  thickness  of  tube  in  inches. 

L  =  distance  between  flanges  in  inches. 
D  =  outside  diameter  of  tube  in  inches. 

Note.—L  must  not  be  greater  than  120  T  -  12. 


BOILEES. 


45 


Steam 
Pressure. 

Distance  Apart 
of  Rings. 

Stress  per  Square  Inch 
on  Metal. 

Lbs. 
100 
120 
160 
200 

Ft.    Ins. 
3      0 
2      8 
2      3 
2       1 

Lbs. 
3,200  to  4,500 
4,000  ,,  5,000 
4,750  ,,  5,700 
5,000  „   6,000 

Strength  of  Riveted  Joints. — The  strength  of  a  riveted 
joint,  as  compared  with  that  of  a  solid  plate,  depends  upon  the 
amount  of  metal  which  is  left  after  drilling  the  rivet  holes,  and 
upon  the  strength  of  the  rivets  to  withstand  shear.  The  strength 
of  the  drilled  plate  is  found  thus — 

P  -  D 


where  P  =  pitch  of  the  rivet. 

D  =  diameter  of  the  hole  drilled  for  the  rivet. 

Example.  —  What  is  the  strength  of  a  |-inch  plate  which  has  been  drilled 
with  1-inch  holes  at  4-inch  pitch  for  a  double-riveted  butt  joint  ? 


--75 


The  answer  is  '75,  so  that  the  plate  is  only  '75  times  as  strong  as  an 
undrilled  plate.     If  we  take  the  case  of  a  plate  which  has  been  drilled  for 
a  single-riveted  lap  joint  —  viz.,  with  1-inch  holes  to  2|-inch  pitch  —  we 
shall  see  that  it  is  still  weaker,  thus  — 
2-25-1  inch 

2-25  5°' 

so  that  the  plate  is  only  '555  times  as  strong  as  an  undrilled  plate. 

The  strength  of  rivets  in  ordinary  practice  is  a  little  greater 
than  that  of  the  drilled  plates.  The  strength  of  rivets  in  a  lap 
joint  is  found  by  the  following  formula  :  — 

A  x  N 

^p  -  ~  =  strength  of  rivets  as  compared  with  the  drilled  plate  ; 

-L     X     JL 

where  A  =  area  of  rivet. 

N  =  number  of  rows  of  rivets. 

P  =  pitch  in  inches. 

T  =  thickness  of  plate  in  inches.- 

Example.  —  What  is  the  strength  of  the  rivets  in  a  f-inch  plate  drilled 
for  1-inch  rivets,  2J-inch  pitch? 

'78  x  1      _ 
2-25  x  -625 
The  rivets  are,  therefore,  '557  times  as  strong  as  the  undrilled  plate. 

In  the  case  of  butt  joints  with  two  straps,  the  rivets  are  in 
double  shear,  and  should  be  twice  as  strong  as  those  in  single 


46  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

shear,  but  as  the  Board  of  Trade  reckon  that  a  rivet  which  has 
to  be  sheared  through  in  two  places  is  only  1'75  times  as  strong 
as  a  rivet  in  single  shear,  this  figure  is  usually  taken,  and  the 
formula  for  finding  the  strength  of  rivets  in  double  shear  is  as 
follows  : — 

A  x  N  x  1-75 


Example.  —  What  is  the  strength  of  the  rivets  in  a  f-inch  plate  drilled 
with  1-inch  holes  at  4  inch  pitch  for  a  double-riveted  butt  joint?  The 
calculation  is 

•785  x  2  x  1-75 

=  */85. 


4  x  -S75 
The  rivets  in  this  case  are  *785  times  the  strength  of  the  undrilled  plate. 

The  size  of  rivets  and  the  pitch  usually  adopted  for  various 
joints  and  for  various  thicknesses  of  plate  are  given  in  Chapter  ii. 

PROPERTIES  OF  STEAM. 

Steam. — Before  discussing  the  evaporative  power  of  boilers, 
chimney  draught,  &c.,  a  few  words  dealing  with  the  raising  of 
steam  may  usefully  be  said.  A  clear  idea  as  to  what  is  meant 
by  the  latent  heat  of  steam  is  essential  to  anyone  calling  himself 
an  engineer. 

If  heat  is  applied  to  a  boiler  which  is  open  to  atmospheric 
pressure,  say  14-7  Ibs.  per  square  inch  (the  atmospheric  pressure 
varies,  and  is  shown  by  the  height  of  the  barometer),  a  thermo- 
meter placed  in  the  water  will  show  a  rise  of  temperature 
corresponding  with  the  amount  of  heat  put  in  until  steam 
begins  to  form.  Under  atmospheric  pressure  in  normal  con- 
ditions this  will  take  place  at  a  temperature  of  21 2°  F.,  but 
before  1  Ib.  of  water  can  be  evaporated  at  this  pressure,  a 
very  large  further  instalment  of  heat  will  require  to  be  put 
into  the  water;  this  heat  will  not  be  shown  by  the  thermo- 
meter. The  heat,  however,  is  there,  and  is  called  the  latent 
heat  of  steam. 

It  is  necessary  to  know  that  the  quantity  of  heat  required  to 
raise  the  temperature  of  1  Ib.  of  pure  water  at  its  greatest 
density  by  1°  F.  is  called  a  British  thermal  unit,  or  B.T.U. 
What  this  thermal  unit  is  equivalent  to  will  be  explained 
later.  Now,  if  there  were  no  such  thing  as  latent  heat  of 
steam,  the  number  of  thermal  units  required  to  raise  1  Ib.  of 
water  from  32°  to  212°  and  turn  it  into  steam  would  be  180. 
As  a  fact,  the  total  heat  which  is  required  to  raise  1  Ib.  of  water 


BOILERS.  47 

from  32°  to  212°,  and  turn  it  into  steam  at  14-7  Ibs.  pressure,  is 
1,146-1  thermal  units.  The  difference — viz.,  966'1 — between  the 
two  sets  of  figures  is  the  latent  heat  in  1  Ib.  of  steam  at  a  pres- 
sure of  14-7  Ibs.  per  square  inch.  The  total  heat  of  steam  at 
H-7  Ibs.  pressure  is  1,146-1  B.T.U. 

If,  instead  of  taking  the  case  of  heat  applied  to  water  at 
atmospheric  pressure,  we  imagine  heat  applied  to  water  under  a 
constant  pressure  of  120  Ibs.  per  square  inch  absolute — by 
absolute  pressure  we  mean  a  pressure  reckoned  from  a  perfect 
vacuum,  and  not  above  the  atmospheric  pressure — the  thermo- 
meter will  rise  steadily  until  it  shows  a  temperature  of  341°  F., 
at  which  temperature  steam  will  begin  to  be  formed ;  but  to 
raise  the  temperature  of  1  Ib.  of  water  from  32°  F.,  and  turn  it 
into  steam  under  a  constant  pressure  of  120  Ibs.,  1,185-4  thermal 
units  will  be  absorbed. 

If  water  is  under  a  constant  pressure  of  200  Ibs.,  and  heat  is 
applied,  the  thermometer  will  rise  to  381-7°  before  steam  begins 
to  be  formed,  and  1,197-8  thermal  units  will  be  required  to  turn 
1  Ib.  into  steam.  If,  on  the  other  hand,  water  is  placed  under  a 
vacuum  and  heat  is  applied,  steam  will  begin  to  form  at  a  tem- 
perature much  below  212°  F.  If  a  closed  glass  vessel,  containing 
water,  is  held  in  the  hand,  the  upper  portion  of  the  vessel  being 
placed  in  communication  with  a  very  effective  air  pump,  and  an 
extremely  good  vacuum  is  formed,  the  temperature  of  the- hand 
is  sufficient  to  cause  the  water  to  boil.  The  temperature  at  which 
water  boils  is,  therefore,  dependent  on  the  pressure  upon  it. 

Table  ix.  gives  the  temperature  of  steam  corresponding  with 
the  pressure,  also  the  total  heat  contained  in  1  Ib.  of  the  steam. 
The  steam  pressures  are  given  in  two  columns,  one  giving  the 
gauge  pressure,  or  pressure  above  the  atmosphere ;  the  other  the 
absolute  steam  pressure. 

Now,  with  regard  to  the  British  thermal  unit,  the  energy 
required  to  raise  a  weight  of  1  Ib.  to  a  height  of  1  foot  is  1 
foot-lb. ;  the  energy  required  to  raise  1  Ib.  10  feet,  or  10  Ibs.  to  a 
height  of  1  foot,  is  10  foot-lbs.  Joule  found  that  772  foot-lbs.  of 
work  were  the  equivalent  of  one  thermal  unit.  That  is  to  say, 
if  772  foot-lbs.  of  work  are  expended  on  a  Ib.  of  water,  by 
violently  agitating  it  with  paddles,  or  by  other  means,  the  tem- 
perature of  the  water  is  raised  by  1°  F.  The  work  of  772* 
foot-lbs.  is  called  the  mechanical  equivalent  of  heat. 

If  the  amount  of  heat  thus  put  into  1  Ib.  of  water,  could 
conveniently  be  utilised,  it  would  be  capable  of  lifting  a  weight 
of  1  Ib.  through  a  height  of  772  feet  (assuming  no  friction) ;  after 

*  More  recent  experiments  show  that  this  figure  is  rather  low,  and  that 
778  foot-lbs.  is  a  more  accurate  figure. 


48 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


doing  this  work  the  water  would  be  back  again  at  its  original 
temperature. 

TABLE  IX. — PROPERTIES  OF  SATURATED  STEAM. 


||3 

0     . 

.«| 

•s 

Hi 

Sfe 

III 

£ 
1 

d 

.SS 

~ 

d)      _  CS 

1 

E 

•s§ 

~ 

Iff 

11 

1 

is 

S 

Iff 

sj 

Is 

"o 

saa 

JJ 

P, 

3J 

1 

III 

sj 

I 

S  "^J  ^ 

^ 

H 

0^ 

•3 

QJ  -^  -<J 

O  ""^ 

1* 

*o 

I 

CO 

H 

> 

CO 

CO 

e-1 

H 

> 

Lbs.  per 
Sq.  In. 

Lbs.  per 
Sq.  In. 

Degs.  F. 

B.T.U. 

Cub.  Ft. 

Lbs.  per 
Sq.  In. 

Lbs.  per 
Sq.  In. 

Degs.  F. 

B.T.U. 

Cub.  Ft. 

1 

102-1 

1112-5 

330-36 

85 

100 

327-9 

1181-4 

4-33 

2 

126-3 

1119-7 

172-08 

90 

105 

333-1 

1182-4 

4-14 

3 

141-6 

1124-6 

117-52 

95 

110 

334-6 

1183-5 

S-97 

4 

153-1 

1128-1 

89-62 

100 

115 

338-0 

1184-4 

3-80 

5 

162-3 

1130-9 

72-66 

105 

120 

341-1 

1185-4 

365 

6 

170-2 

1133-3 

61-21 

110 

125 

344-2 

1186-4 

3-51 

7 

176-9 

1135-3 

52-94 

115 

130 

347-2 

1187-5 

3-38 

8 

182-9 

1137-2 

46-69 

120 

135 

350-1 

1188-2 

3-27 

9 

188-3 

1138-8 

41-79 

125 

140 

352-9 

1189-0 

3-16 

10 

193-3 

1140-3 

37-84 

130 

145 

355-6 

1189-9 

3-06 

11 

197-8 

1141-7 

34-62 

135 

150 

358-3 

11907 

2-96 

12 

202-2 

1143-0 

31-88 

140 

155 

361-0 

1191-5 

2-87 

13 

205-9 

1144-2 

29-57 

145 

160 

364-3 

11922 

2-79 

14 

209-6 

1145-3 

27-61 

150 

165 

3660 

1192-9 

2-71 

14-7 

212-0 

1146-1 

26-36 

155 

170 

36S-2 

1193-7 

2-63 

15 

213-1 

1146-4 

25-85 

160 

175 

370-8 

1194-4 

2-56 

1 

16 

216-3 

1147-4 

24-24 

165 

ISO 

372-0 

1195-1 

2-49 

2 

17 

219-6 

1148-3 

22-89 

170 

185 

375-3 

1195-8 

2-43 

3 

18 

222-4 

1149-2 

21-70 

175 

190 

3775 

1196-5 

2-37 

4 

19 

225-3 

1150-1 

20-64 

180 

195 

379-7 

1197-2 

2-31 

5 

20 

228-0 

1150-9 

19-72 

185 

200 

381-7 

1197-8 

2-26 

10 

25 

240-1 

1154-6 

15-99 

190 

205 

383-8 

1198-4 

2-21 

15 

30 

250-4 

1157-8 

13-46 

195 

210 

385-8 

1199-1 

2  16 

20 

35 

259-3 

1160-5 

11-65 

200 

215 

387-8 

1200-1 

2-12 

25 

40 

267-3 

1162-9 

10-27 

205 

220 

389-9 

1200-3 

2-08 

30 

45 

274-4 

1165-1 

9-18 

235 

250 

401-1 

1203-7 

1-82 

35 

50 

281-0 

1167-1 

8-31 

285 

300 

417-5 

1208-7 

1-53 

40 

55 

287-1 

1169-0 

7-61 

335 

350 

430-1 

1212-6 

1-32 

45 

60 

292-7 

1170-7 

7-01 

385 

400 

449-9 

1217-1 

1-16 

50 

65 

298-0 

1172-3 

6-49 

435 

450 

456-7 

12207 

1-05 

55 

70 

302-9 

1173-8 

6-07 

485 

500 

467-6 

1224-0 

•94 

60 

75 

307-5 

1175-2 

5-68 

585 

600 

487-0 

1229-9 

•80 

65 

80 

312-0 

1176-5 

5-35 

685 

700 

504-1 

1235-1 

•68 

70 

85 

316-1 

1177-9 

5-05 

785 

800 

519-5 

1239-8 

•60 

75 

90 

320-0 

1179-1 

4-79 

885 

900 

533-6 

1244-2 

•54 

80 

95 

324-1 

1180-3 

4-55 

985 

1000 

546-5 

1248-1 

•49 

BOILERS.  49 

Steam  in  contact  with  water  is  called  saturated  steam ;  this  is 
not  the  same  thing  as  the  steam  being  wet.  Wet  steam  is  steam 
in  which  particles  of  water  have  become  entrained  through  the 
steam  leaving  the  water  at  too  great  a  velocity,  or  through  the 
temperature  having  become  lowered,  and  some  of  the  steam 
having  turned  back  into  water.  Dry  saturated  steam  is  steam 
which  is  free  from  particles  of  water.  Superheated  steam  is 
steam  to  which  further  heat  has  been  added  after  it  has  been 
formed.  Such  steam,  if  placed  in  contact  with  water,  will  absorb 
a  part  of  it,  turning  the  water  into  steam ;  the  temperature  of 
the  steam  falls  slightly  during  the  process.  This  addition  of 
heat  to  the  steam,  or  superheating  it,  after  it  has  been  formed, 
does  not  increase  its  pressure  (apart  from  the  pressure  due  to 
increase  of  volume),  and  unlike  latent  heat,  it  is  shown  by  a 
thermometer. 

Evaporation  of  a  Boiler. — The  evaporation  of  a  boiler  as 
given  by  the  makers  is  usually  in  terms  of  pounds  of  water  evapo- 
rated "from  and  at  212°  F."  This  means  that  so  many  pounds 
of  water  will  be  evaporated,  if  it  is  fed  into  the  boiler  at  a 
temperature  of  212°,  and  is  evaporated  at  atmospheric  pressure. 
If,  however,  the  water  is  fed  into  the  boiler  at  a  lower 
temperature,  and  is  evaporated  under  a  considerable  pressure, 
the  evaporation  of  the  boiler  will  be  less  than  that  given  by  the 
maker.  Thus,  if  water  is  fed  into  the  boiler  at  a  temperature  of 
60°  F.,  and  is  evaporated  under  a  pressure  of  120  Ibs.,  the  amount 

of  water  evaporated  will  be  only  —  that  given  by  the  makers. 
If  the  water  is  fed  in  at  150°  F.,  and  evaporated  at  120  Ibs.,  the 

evaporation  will  be  about  — .     If  fed  in  at  32°  F.,  and  evaporated 

10-0 
at  200  Ibs.  pressure,  the  evaporation  will  be  ^n^r  of  that  given. 

The  formula  to  enable  one  to  find  out  what  the  actual  evapora- 
tion will  be,  if  the  evaporation  "from  and  at"  is  known,  is  as 
follows : — 

B  +  32  -  C 


A  = 


966 


where  A  =  the  factor  by  which  the  Ibs.  given  as  "from  and  at" 

are  divided. 

B  =  the  total  heat  of  the  steam  at  the  working  pressure. 
C  =  the  temperature  of  the  feed  water. 

Example. — If  the  makers  say  that  a  boiler  will  evaporate  1,200  Ibs.  of 
water  per  hour  from  and  at  212°,  how  many  Ibs.  will  it  evaporate  if  the 
working  pressure  is  to  be  120  Ibs.  (or  135  Ibs.  absolute)  and  the  temperature 

4 


50  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

of  the  feed  is  only  60°  F.  ?  The  reader  will  see  from  Table  ix.  that  the 
total  heat  of  steam  at  120  Ibs.  pressure  (above  atmosphere)  is  1,188  B.T.U. 
The  formula  then,  in  actual  figures,  is 

.  _  1,188  4-  32  -  60  _  1-9 
966 

The  answer  is  1*2,  so  that,  if  we  divide  the  1,200  Ibs.  given  by  the 
makers  by  1'2,  we  get  1,000  Ibs.,  which  is  the  amount  of  water  the  boiler 
will  evaporate  under  the  conditions  given. 

If  the  actual  evaporation  is  known,  and  it  is  desired  to 
ascertain  what  the  equivalent  evaporation  "from  and  at  212°" 
would  be,  the  number  of  Ibs.  actually  evaporated  is  multiplied 
by  the  factor  obtained  by  the  formula  given  above. 

Coal  and  its  Evaporative  Power. — We  have  seen  what  a 
certain  number  of  B.T.U.  are  capable  of  doing  in  the  way  of 
raising  steam ;  this  information,  however,  is  of  but  little  practical 
use  to  us,  unless  we  know  how  many  B.T.U.  we  can  get  from  a 
pound  of  coal.  A  pound  of  coal  burnt  in  the  laboratory  gives 
from  13,000  to  15,000  B.T.U.  A  pound  of  pure  carbon  gives 
14,500  B.T.U.,  but  coal  contains  other  constituents  than  carbon, 
such  as  hydrogen,  oxygen,  nitrogen,  sulphur,  and  ash.  The 
B.T.U.  values  of  1  Ib.  of  some  of  the  best  known  coals,  and  of 
petroleum,  are  approximately  as  follows  : — 

Powell  Duffryn,  ....  15,500 

Nixon's  Navigation,    ....  15,000 

Newcastle, 14,820 

Derbyshire,  j  ,„  860 

Yorkshire,    \ ld'bb 

Scotch 13,500 

Coke, 12,820 

Petroleum, 

Steam  coals  are  those  which  contain  a  large  proportion  of  fixed 
carbon  and  a  small  proportion  of  volatile  constituents.  They 
burn  without  giving  off  much  gas.  A  coal  which  fulfils  these 
conditions  to  the  fullest  extent  is  anthracite ;  the  largest  pro- 
portion of  coal  is,  however,  bituminous  or  smoky  coal. 

Powell  Duffryn  is  almost,  but  not  quite,  pure  anthracite ;  it 
contains  about  88 '24  per  cent,  of  carbon.  Nixon's  Navigation  is 
semi-anthracite.  The  others  given  above  are  bituminous ;  they 
contain  from  75  to  83  per  cent,  of  carbon,  the  remainder  consisting 
of  various  volatile  constituents  and  ash.  Coke  contains  from  86 
to  88  per  cent,  of  carbon. 

Coals  are  sometimes  divided  into  caking  and  non-caking  coals; 
the  former  soften  when  heated,  and  form  a  spongy  mass ;  in 
non-caking  coals  the  particles  remain  separate  and  allow  the  air 


BOILERS.  51 

to  pass  between  them.  Cannel  coal,  which  is  very  rich  in  volatile 
constituents,  is  used  for  producing  gas  for  lighting  purposes. 

The  ash  in  Powell  Duffryn  coal  is  about  3-26  per  cent.  In 
Nixon's  Navigation  it  is  about  7  per  cent.,  while  in  inferior  coals 
it  may  amount  to  10  per  cent,  or  more. 

One  Ib.  of  coal,  yielding  14,007  B.T.U.,  is  theoretically  capable 
of  evaporating  14-5  Ibs.  of  water  from  and  at  212°  F.  In  actual 
practice  1  Ib.  of  coal  evaporates  between  8  and  10  Ibs.  of  water. 
Why,  it  may  be  asked,  this  great  difference  1  In  the  first  place 
there  is  the  correction  to  be  made  for  the  fact  that  the  feed  may 
not  be  so  hot  as  212°,  and  for  the  fact  that  the  water  will,  of 
course,  be  evaporated  at  a  much  higher  pressure  than  that 
corresponding  with  212°.  The  formula  for  making  the  necessary 
correction  has  already  been  given.  Then  there  is  the  loss  due 
to  the  heat  which  passes  away  in  the  gases  to  the  chimney. 
The  temperature  of  these  gases  must  not  be  lower  than  that  of 
the  steam  in  the  boiler,  otherwise  they  would  be  harmful.  The 
losses  from  this  source  are  probably  from  12  to  15  per  cent.,  and 
may  easily  be  greater.  The  other  losses  are  those  due  to  incom- 
plete combustion,  radiation,  &c. 

An  economiser,  which  will  be  described  later,  will  extract  and 
make  use  of  some  of  the  heat  of  the  waste  gases,  and  if  the 
boiler  itself  extracts  70  per  cent,  of  the  heat  theoretically 
contained  in  the  coal  it  may  be  considered  efficient. 

When  boilermakers  say  that  a  boiler  will  evaporate  so  many 
Ibs.  of  water  per  Ib.  of  combustible,  they  mean  that  the  weight 
of  the  unconsumable  ash  must  be  deducted  from  the  weight  of 
the  coal  burnt. 

Rate  of  Combustion. — The  number  of  pounds  of  coal  which 
can  be  burnt  on  each  square  foot  of  grate  area  depends  on  the  kind 
of  coal  and  upon  the  draught.  The  draught  given  by  a  chimney 
depends  upon  its  height,  assuming  that  its  area  is  sufficiently 
large  to  carry  off  the  gases.  Draught  caused  by  the  height  of 
the  chimney  alone  is  called  natural  draught*;  if  a  fan  is  put  inside 

*The  formula  to  enable  the  actual  draught  to  be  ascertained  is  as 
follows:-  7.6  7.9 

\A  +  461/      \B  +  46lJ  ; 

where  H  =  height  of  chimney  in  feet. 
A  =  temperature  of  external  air. 
B  =  temperature  of  gases  in  chimney. 
D  =  draught. 

Example. — What  will  be  the  draught  in  a  chimney,  100  feet  high,  when 
the  temperature  outside  is  60°,  and  inside  400°  ? 

100  (      7'6       \-  f       7'9       \  =  -542. 
V60  +  461/     V400  +  4 

The  answer  is  '542  inch. 


52  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

the  flue  to  increase  the  draught,  the  latter  is  called  induced 
draught ;  if  a  fan  is  employed  to  increase  the  pressure  of  air 
in  the  stokehold  of  the  furnace  the  draught  is  called  forced 
draught. 

The  intensity  of  the  draught  is  spoken  of  as  so  many  inches, 
or  parts  of  an  inch,  of  water.  This  means  that  if  a  U-shaped 
tube  is  partly  filled  with  water,  and  one  leg  of  the  U  is  placed  in 
communication  with  the  chimney,  the  other  leg  being  open  to 
the  atmosphere,  the  water  in  the  leg  connected  to  the  chimney 
rises  above  the  level  of  the  water  in  the  other  leg.  If  the 
difference  is  J  inch  the  draught  is  said  to  be  equal  to  '25  inch  of 
water. 

With  a  properly  constructed  chimney,*  100  feet  high,  and  with 
gases  at  a  temperature  of  400°  above  that  of  the  atmosphere 
outside,  say  60°  F.,  a  draught  of  about  -54  of  an  inch  is  obtained. 
When  fans  are  employed  to  give  forced  draught  the  draught  is 
usually  between  1  and  4  inches.  If  the  draught  is  forced  to  too 
great  an  extent  it  is  injurious  to  the  boiler,  owing  to  the  intense 
heat  caused. 

We  have  said  that  the  amount  of  coal  that  can  be  burnt  on 
each  square  foot  of  grate  area  depends  on  the  kind  of  coal  and 
on  the  draught.  The  following  are  approximately  the  amounts 
of  coal  which  can  be  burnt  per  square  foot  of  grate,  with  a 
draught  of  '5  inch,  and  with  hand  firing. 

Coke,     ....  1 1-12  Ibs.  per  hour. 

Nixon's  Navigation,       .  18-20     ,,         ,, 

Powell  Duffryn,      .         .  20-23     ,,         ,, 

Bituminous  coal,    .         .  20-32     ,,         ,, 

The  nearer  the  coal  approaches  the  qualities  of  anthracite  the 
greater  is  the  draught  required.  Bituminous  coal  requires  less 

*  A  good  empirical  formula  for  determining  the  most  suitable  area  for  a 
chimney  is  the  following  : — 

A-    -JL', 

^71' 

where  A  =  area  in  square  feet. 

W  =  weight  of  coal  burnt  per  hour. 
H  =  height  of  chimney  in  feet. 

Example. — What  must  be  the  area  of  a  chimney,  150  feet  high,  for 
boilers  burning  2,000  Ibs.  of  coal  per  hour  ? 

A  =  16^/750  or  A  =  16  x  12-24  =  10'2  feet* 

The  answer  is  10 '2  square  feet. 

A  rule  often  worked  to  is  6  square  feet  of  chimney  area  for  every  30 
feet  x  8  feet  Lancashire  boiler  up  to  four  boilers  ;  beyond  this  5  square 
feet  for  every  additional  boiler. 


BOILERS.  53 

draught;  with  a  draught  of  '25  inch,  from  16  to  20  Ibs.  of 
bituminous  coal  can  be  burnt  per  square  foot  of  grate  area  per 
hour. 

The  ratio  of  grate  area  to  heating  surface  varies,  or  should 
vary,  according  to  the  kind  of  fuel  to  be  consumed  and  the 
draught.  The  ordinary  ratios  in  Lancashire,  water-tube,  and 
locomotive  boilers  are  approximately  as  follows  : — 

Grate    Heating 
Area.    Surface. 

Locomotive  boiler, 1        23-28 

Water-tube  boiler  (Babcock  type),    .         .         1         50-70 
Locomotive  boiler,     .....         1         60-80 

The  evaporation  per  square  foot  of  heating  surface  varies  con- 
siderably in  different  types  of  boiler.  Makers  of  Lancashire  and 
Cornish  boilers  usually  allow  about  1  square  foot  of  heating 
surface  for  every  5  to  10  Ibs.  of  water  to  be  evaporated  from 
and  at  212"  F.  Makers  of  the  Babcock  boiler  allow  about 
1  square  foot  for  every  2J  to  3|  Ibs.  to  be  evaporated.  In 
locomotive  boilers  with  a  strong  draught  1  square  foot  of  heating 
surface  will  evaporate  from  10  to  17  Ibs.  of  water.  The  actual 
evaporation  depends  on  the  amount  of  coal  properly  burned  in 
relation  to  the  heating  surface,  and  upon  the  degree  of  effective- 
ness of  the  heating  surface. 

In  order  that  coal  may  be  burnt  to  the  best  advantage,  the 
right  amount  of  air  must  be  supplied,  and  there  must  be 
sufficient  space  in  which  combustion  can  be  carried  out.  If 
insufficient  air  is  supplied,  or  if  combustion  is  interfered  with  by 
contact  with  a  cool  surface,  incomplete  combusion  takes  place, 
and  carbon  monoxide  or  CO  is  formed.  Now,  if  1  Ib.  of  carbon 
is  improperly  burned  so  that  it  forms  CO,  it  will  give  only 
about  4,330  B.T.U.,  as  against  14,500  B.T.U.,  which  it  gives  if 
properly  burnt  so  as  to  form  carbon  dioxide  or  C02.  If,  on  the 
other  hand,  too  much  air  is  admitted  to  the  furnace,  the  tem- 
perature is  reduced,  and  some  of  the  heat  given  out  by  the  coal 
is  wasted.  Smoke  and  soot  are  produced  by  the  incomplete 
combustion  of  the  carbon. 

The  number  of  heat  units  which  can  be  transmitted  through 
a  steel  boiler  plate  depends  principally  upon  the  difference  in 
temperature  of  the  gases  on  one  side  of  the  boiler  plate  and  of 
the  water  on  the  other  side,  and  in  a  less  degree  upon  the  thick- 
ness of  the  plate. 

Mr.  Blechynden,  in  a  paper  read  before  the  Institute  of  Naval 
Architects  some  years  ago,  gave  the  results  of  a  series  of  ex- 
haustive experiments  which  he  made  as  to  the  transmission  of 
heat  through  plates  of  various  thicknesses  and  with  different 


54 


MECHANICAL    ENGINEERING   FOR    BEGINNERS. 


degrees  of  temperature  on  the  two  sides  of  the  plate.      The 
following  are  some  of  the  results  obtained  : — 


Difference  of 
Temperature  of 
the  Two  Sides 
of  Plate. 

Heat  trans- 
mitted per  1° 
difference 
per  square  foot 
per  hour. 

848 
1,013 
1,278 

•   12-78 
•   15-26 
•  20-9 

'Plate  1-1875  inch  thick. 

J  »                                         5> 

626 
1,058 
1,233 

1089 
19-18 
21-92 

Plate    '75      inch  thick. 
»                   » 

563 
1,148 

11-90 
25-7 

Plate    -5625  inch  thick. 

503 
723 

893 

11-81 
1655 
20-65 

Plate     25      inch  thick. 
>  »                   i  > 

»                   >» 

738 
1,083 

16-46 
25-48 

Plate    -125    inch  thick. 

In  a  paper  read  before  the  same  society  a  year  previously,  Mr. 
(now  Sir  John)  Durston  gave  the  results  of  some  experiments 
undertaken  to  show  the  effect  of  grease  on  the  surface  of  boiler 
plates ;  the  experiments  showed  that  a  film  of  grease  caused  a 
most  astonishing  decrease  in  the  number  of  heat  units  which 
could  be  transmitted  through  a  plate  of  a  given  thickness,  and 
with  a  given  difference  of  temperature  on  its  two  sides. 

Peed  Water  for  Boilers  and  Boiler  Compositions. — It 
is  important  that  the  feed  water  for  boilers  should  be  as  free 
from  impurities  as  possible.  Water  usually  contains  a  certain 
amount  of  lime,  magnesia,  and  other  impurities ;  these  are 
thrown  down  by  the  heat  in  the  form  of  sediment,  which  be- 
comes extremely  hard,  and  has  an  injurious  effect  on  the  boiler. 
The  incrustation  acts  as  a  non-conductor  of  heat,  so  that  the 
water  is  unable  to  take  away  the  heat  from  the  steel  boiler 
plates,  and  the  metal  may  in  consequence  get  unduly  hot ;  this 
in  turn  may  cause  the  furnace  crown  to  collapse,  or  may  give 
rise  to  undue  expansion,  and  thus  set  up  dangerous  stresses  in 
the  boiler.  The  non-conducting  properties  of  this  scale  may  also 
cause  a  serious  falling  off  in  the  evaporative  power  of  the  boiler. 


BOILERS.  55 

It  is  considered  that  an  incrustation  J.  inch  thick  causes  a 
falling  off  of  15  per  cent,  in  the  evaporation,  and  consequently 
15  per  cent,  of  the  coal  is  wasted. 

As  previously  mentioned,  the  impurities  in  water  are  thrown 
down  by  heat.  This  fact  is  made  use  of  in  the  Niclausse  boiler,  in 
which  the  feed  water  is  admitted  to  the  steam  space  in  the  form 
of  spray  over  a  suitable  tray  provided  with  a  sludge  pipe.  The 
impurities  fall  into  the  tray  in  the  form  of  sludge,  and  the  latter 
is,  or  should  be,  periodically  blown  out.  The  Boby  feed-water 
heater  and  detartariser  acts  on  the  same  principle.  Various 
mechanical  niters  are  made  for  removing  mud  and  impurities 
merely  held  in  suspension.  In  these  provision  is  made  for 
cleaning  the  filtering  material,  either  by  blowing  through  steam 
or  by  reversing  the  direction  of  the  current  of  water. 

Various  boiler  compositions  to  prevent  incrustation  are  sold ; 
the  effect  of  these  is  to  prevent  the  sediment  forming  a  hard 
scale,  but  these  compositions  should  not  be  used  unless  the  water 
has  been  analysed,  and  the  user  has  reasonable  grounds  for 
believing  that  the  composition  will  do  what  is  claimed  for  it. 
Most  of  the  compositions  contain  tannic  acid.  This  acid  is 
effective  in  cases  where  the  water  contains  carbonate  of  lime 
and  magnesia,  but  if  used  in  excess  is  injurious  to  the  boiler 
plates.  Soda  is  useful  when  the  water  contains  sulphate  of 
lime  or  acids,  but  it  causes  a  boiler  to  prime  if  used  in  excess. 
The  right  course,  if  the  water  is  hard  or  impure,  is  to  treat  it 
chemically,  or  by  filtration  before  it  enters  the  boiler. 

Testing  Boilers. — Before  a  boiler  leaves  the  makers'  works 
it  is  tested  by  hydraulic  pressure  for  tightness  and  strength. 
The  pressure  usually  employed  is  50  per  cent,  greater  (sometimes 
100  per  cent,  greater)  than  the  working  pressure  for  which  the 
boiler  has  been  constructed.  After  a  boiler  has  been  supplied 
and  fixed,  the  purchaser  may  desire  to  test  it  for  its  evaporative 
performance.  This  is  done  by  weighing  or  measuring  the  water 
pumped  in,  the  coal  consumed,  and  the  weight  of  the  ashes 
removed.  Care  is  taken  to  see  that  the  level  of  the  water  is  the 
same  at  the  end  as  at  the  beginning  of  the  trial,  and  that  the 
thickness  of  the  fire  is  approximately  the  same  at  the  commence- 
ment and  end  of  the  trial. 

The  results  of  such  a  trial  are  usually  tabulated  in  the  manner 
shown  by  Table  x.  Trials  are  often  carried  out  in  a  more  elabo- 
rate manner  than  is  indicated  by  the  table.  For  instance,  the 
coal  may  be  analysed  to  ascertain  the  number  of  B.T.U.  yielded 
by  it.  The  waste  gases  may  also  be  analysed,  and  the  steam  be 
tested  for  dryness.  Steam  is  tested  for  dryness  usually  by 
means  of  a  throttle  calorimeter.  The  principle  of  the  calori- 


56 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


meter  is  this : — When  steam  passes  through  a  constricted 
opening,  it  is  said  to  be  wire-drawn,  and  becomes  slightly 
superheated,  the  amount  of  the  superheat  depending  on  the 
dryness  of  the  steam.  Now,  the  amount  by  which  really  dry 
steam  becomes  superheated  being  known,  if  the  amount  of 
superheat  imparted  to  the  steam  under  test  is  ascertained,  it 
is  not  difficult  to  calculate  the  percentage  of  moisture  in  it. 

Trial  of  a  Water-tube  Boiler  for  a  250  I.H.P. 
Compound  Non- condensing  Engine. 


22/8/06. 
8  hours. 

1,506  square  feet. 
28  square  feet. 
53-8  to  1. 
180  Ibs. 
158°  F. 
4,190-4. 
297-5. 
3,892-9. 
7-1. 

18-7  Ibs. 
38,552  Ibs. 
4,819  Ibs. 

9-2  Ibs. 
10-3  Ibs. 

11-1  Ibs. 
450°. 
•5  inch. 

71  per  cent. 

Note. — If  an  economiser  is  fixed  in  connection  with  the  boiler,  its 
heating  surface  is  given,  and  the  temperature  of  the  water  and  gases 
before  entering  and  after  leaving  are  also  given.  If  a  superheater  is 
used,  its  heating  surface  is  given,  and  the  temperature  of  the  steam  as 
it  leaves  the  boiler  is  also  noted. 

Results  similar  to  those  given  in  the  table  would  be  expected  from  a 
good  Lancashire  boiler,  but  the  heating  surface  would  be  less,  and  the 
temperature  of  the  flue  gases  would  probably  be  higher. 


Date  of  test,       ...... 

Duration  of  test,        ..... 

Heating  surface, 

Grate  area,         ...... 

Ratio  of  heating  surface  to  grate  area, 
Average  gauge  pressure,    .... 

Temperature  of  feed  water, 

Pounds  of  coal  burnt,         .... 

„          refuse,        ..... 

„          combustible,      .... 
Per  cent,  of  ashes,      ..... 
Coal  burnt  per  square  foot  of  grate,  . 
Total  water  evaporated,      .... 
Water  evaporated  per  hour, 
Water  evaporated  per  pound  of  coal  under 

actual  conditions,         .... 
Water  evaporated  per  pound  of  coal  from 

and  at  212°  F.,    .  .         . 

Water  evaporated  per  pound  of  combustible 

from  and  at  212°  F.,   . 
Temperature  of  flue  gases, 
Draught  in  inches  of  water, 
Efficiency  of  boiler,   assuming  a  basis  of 

14,000  B.T.U.  per  pound  of  coal, 


57 


CHAPTER  IV. 


STEAM-RAISING   ACCESSORIES. 


WE   will   now   consider    some    accessories   which   are   used   in 
connection  with  steam  boilers. 

Pumps. — In  order  to  force  water  into  a  boiler  under  steam 
pressure,  a  pump  or  injector  is  required.  When  the  boiler 
is  used  for  driving  a  slow-speed  engine,  the  latter  is  usually 
provided  with  its  own  pump,  driven  from  an  eccentric  fitted  on 
the  shaft,  or  from  some  reciprocating  part  of  the  engine,  so  that 
at  every  stroke  of  the  engine  a  small  quantity  of  water  is  forced 
into  the  boiler.  A  by-pass  arrangement  is  provided,  so  that  any 
excess  water  may  be  returned  to  the  hot  well.  When  the  engine 


Fig.  2L — Worthington  Duplex  feed  pump. 

is  of  the  high-speed  type,  or  when  the  installation  consists  of 
several  engines  or  turbines,  a  separate  pump  is  usually  employed. 
This  pump  may  be  driven  by  steam  or  by  an  electric  motor ;  by 
running  it  faster  or  slower  a  greater  or  smaller  quantity  of  water 
can  be  fed  in  to  the  boilers,  and  thus  the  water-level  be  kept 
constant. 

When  a  steam  pump  is  employed,  a  very  common  form  is  that 
known  as  the  Duplex  direct-acting  pump,  shown  by  Fig.  21.  In 
this  pump  there  are  two  steam  and  two  water  cylinders  placed 
horizontally  side  by  side  ;  the  steam  piston  is  connected  by  a  rod 


58 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


to  the  water  piston  or  plunger.  The  area  of  the  steam  piston  is 
usually  twice  that  of  the  water  piston,  consequently  the  former 
is  able  to  drive  the  latter.  The  motion  of  the  rod  of  one  steam 
cylinder  gives  the  necessary  motion  to  the  valve  of  the  other 
cylinder.  The  water  enters  through  the  space  marked  "inlet," 
is  forced  through  the  upper  valves  into  the  space  marked  D,  and 
from  thence  it  descends  through  a  passage  placed  between  the 
two  pumps  to  the  common  discharge,  D  D. 

The  Duplex  pump  is  inexpensive  and  reliable,  but  is  wasteful 
of  steam,  as  the  steam  is  admitted  right  to  the  end  of  every 
stroke,  and  the  steam  clearance  spaces  are  large. 

A  much  more  economical  form  of  direct-acting  steam  pump  is 
the  Weir  (Fig.  2 la).  In  this  pump  there  is  usually  one  steam 
cylinder  and  one  water  cylinder  arranged  singly,  as  shown  by 
the  illustration,  or  in  pairs.  The  valve  is  arranged  to  cut  off 
steam  when  the  piston  has  travelled  about  75  per  cent,  of  its 
stroke,  so  that  during  the  last  25  per  cent,  of  the  stroke  the 
steam  is  used  expansively.  Provision  is  made  for  admitting 
steam  by  by-passes  during  the  whole  of  the  stroke  when  the 
pump  is  first  started ;  when  the  pump  is  well  under  weigh  the 
by-pass  valves  are  closed.  There  are  two  steam  valves,  one  main 
and  one  auxiliary  valve ;  the  latter  is  actuated  by  a  tappet 
motion  from  the  pump  rod,  and  admits  steam  to  either  side 
of  the  main  valve ;  the  latter  is  alternately  driven  backwards 
and  forwards  by  the  steam,  and  so  controls  the  action  of  the 
pump. 

The  following  is  a  table  giving  some  of  the  standard  sizes  of 
the  Weir  pumps,  having  one  steam  cylinder  and  one  water 
barrel : — 

TABLE  X. 


Diameter 

of 

Diameter 

Length 

Strokes 

Size 

Size 

Gallons 

Steam 
Cylinder. 

of 
Pump. 

of 
Stroke. 

Minute. 

of 

Suction. 

of 
Delivery. 

Hour. 

Inches. 

Inches. 

Inches. 

Inches. 

Inches,   i 

5 

84 

6 

44 

u 

1 

515 

6 

4 

8 

40 

if 

14 

810 

6* 

** 

10 

36 

2 

14 

1,160 

5 

12 

30 

24 

2 

1,450 

8 

54 

12 

30 

24 

2 

1,730 

81 

6 

12 

28 

3 

24 

1,950 

9* 

7 

18 

26 

H 

3 

3,700 

104 

8 

18 

24 

3£ 

4,450 

12 

9 

24 

24 

44 

tt 

7,540 

STEAM-RAISING   ACCESSORIES.  59 

If  the  reader  checks  the  number  of  gallons  delivered  according 
to  the  table  by  the  formula  given  later,  he  will  find  that  a  some- 
what small  allowance  has  been  made  for  slip.  If,  however,  the 
estimated  slip  should  be  exceeded,  the  number  of  gallons  given 


Fig.  21a. — Weir  feed  pump. 


60 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


above  can  be  obtained  by  running  the  pumps  a  little  faster,  as 
the  piston  speed  indicated  by  the  figures  is  very  moderate 
indeed.  The  Weir  pump  is  also  made  in  the  compound  form 
— i.e.,  having  one  H.P.  cylinder,  one  L.P.  cylinder,  and  two 
water  cylinders. 

Another  very  reliable  form  of  direct-acting  pump  is  the 
"  Deane,"  made  by  the  Pulsometer  Engineering  Company,  but 
in  this  pump  the  steam  is  not  used  expansively. 

Still  another  form  of   steam   pump  is  that  of  the  flywheel 
type,  as  shown  by  Fig.  22,  and  made  by  Messrs.  Cameron,  of 
Is,  and  others.     In  this  pump  there  are  one  or  more  steam 

cylinders  with  corresponding 
water  cylinders.  The  valve 
control]  ing  the  supply  of  steam 
to  each  cylinder  is  driven  from 
an  eccentric  mounted  on  the 
shaft  carrying  the  flywheel. 
With  this  form  of  pump  the 
admission  of  steam  need  not 
be  continued  until  the  end  of 
the  stroke,  so  that  it  may  be 
used  expansively ;  further, 
compound  cylinders  may  be 
used.  The  objection  to  the 
flywheel  form  of  pump  is  that, 
should  any  temporary  ob- 
struction occur  in  the  delivery 
pipes,  the  energy  stored  in 
the  flywheel  may  cause  an 
undue  rise  of  pressure  in  the 
pipes. 

In  electric  generating 
stations  electrically  -  driven 
feed  pumps  are  sometimes 
employed ;  a  small  electric 
motor  running  at  a  high  speed 
drives  the  pump  through  suitable  gearing  at  a  moderate  speed. 
There  are,  however,  certain  electrical  difficulties  in  regulating 
the  speed  of  the  motor  through  a  sufficiently  wide  range  of 
speed,  and  a  by-pass  on  the  delivery  side  of  the  pump  is  a 
wasteful  arrangement. 

The  number  of  gallons  of  water  which  should  be  discharged 
by  a  double-acting  pump,  if  there  were  no  slip,  can  be  found 
thus — 


Fig.  22. — Cameron  pump. 


STEAM-RAISING   ACCESSORIES.  61 

A  x  L  x  N 


277 

where  A  =  area  of  pump  bucket  or  plunger  in  inches. 
L  =  length  of  stroke  in  inches. 
N  =  number  of  effective  strokes  per  minute. 
G  =  gallons  per  minute. 

To  obtain  the  number  of  gallons  delivered  per  hour,  the  result 
must  be  multiplied  by  60. 

If  the  pump  is  of  the  ram  type  which  draws  in  water  once 
during  every  two  strokes,  the  number  of  effective  strokes  is 
reduced  by  half.  If  the  pump  has  more  than  one  barrel,  the 
result  obtained  by  the  above  formula  must  be  multiplied  by  the 
number  of  barrels.  If  it  is  desired  to  find  the  number  of 
pounds  of  water  discharged  per  minute,  the  number  of  gallons 
must  be  multiplied  by  10. 

The  formula  does  not  take  into  account  slip — i.e.,  the  per- 
centage of  water  which  finds  its  way  back  while  the  valves  are 
closing,  or  which  leaks  past  the  bucket  or  ram.  It  is,  therefore, 
necessary  to  make  an  allowance  for  this,  and  in  the  case  of  the 
average  boiler  feed  pump  it  is  best  to  assume  that  only  90  per 
cent,  of  the  amount  which  should  theoretically  be  delivered  is 
actually  pumped.  It  should  be  stated  that  there  are  277*27  cubic 
inches  in  a  gallon,  while  in  the  formula  the  figure  is  given  as 
277.  The  latter  figure  is  given  for  convenience  of  calculation ; 
it  is  sufficiently  accurate,  considering  that  a  margin  must,  in  any 
case,  be  allowed  for  slip. 

Speeds  up  to  100  feet  per  minute  are  suitable  for  the  buckets 
or  plungers  of  boiler  feed  pumps. 

Injectors. — In  locomotives  the  water  is  usually  supplied  by 
means  of  an  injector.  This  instrument  was  invented  by  a  French 
engineer  named  Giffard ;  by  its  use  a  boiler  can  feed  itself  with 
water  without  the  intervention  of  a  force  pump. 

An  injector  is  shown  diagrammatically  in  Fig.  23.  The  prin- 
ciple upon  which  it  works  is  this : — The  steam  nozzle  is  surrounded 
by  cool  water  at  A ;  when  the  steam  passing  through  the  pipe 
B  meets  this  water  it  condenses  and  forms  a  partial  vacuum ; 
this  causes  the  steam  in  the  pipe  B  to  travel  at  a  great  velocity, 
in  order  to  fill  up  the  vacuum  as  the  steam  is  at  the  full  boiler 
pressure ;  the  vacuum  also  causes  the  water  in  pipe  C  to  rush 
in  at  a  great  velocity,  the  water  being  driven  in  by  the  pressure 
of  the  atmosphere.  The  velocity  acquired  by  the  column  of 
steam  and  water  is  so  great  that  it  causes  the  pressure  on  the 
under  side  of  the  check  valve  to  be  higher  than  the  pressure 


62 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


Steam 


above  it,  the  check  valve  opens,  the  condensed  steam  and  water 
enter  the  boiler,  and  so  keep  it  supplied. 

A  modern  injector  (made  by  Messrs.  Holden  &  Brooke,  of 
Manchester)  is  shown  by  Fig.  24.  The  injector  is  of  the  one- 
movement  type — that  is  to  say,  the  admission  of  steam  and 
water  is  simultaneously  regulated  by  the  one  handle. 

On  first  starting  an  injector,  before  the  column  of  condensed 

steam  and  water  has  acquired 
enough  energy  to  open  the 
check  valve,  it  passes  away 
through  the  overflow  outlet, 
but  as  soon  as  the  injector  gets 
to  work  the  overflow  ceases. 

An  injector  which  is  required 
to  lift  its  own  water,  and  to 
work  with  fairly  high  steam 
pressures,  will  not  work  satis- 
factorily if  the  temperature  of 
the  water  at  A  is  higher  than 
about  80°  F.  If,  however,  the 
injector  is  arranged  so  that  the 
water  flows  into  it  by  gravity, 
injection  water  at  a  higher 
temperature  may  be  used, 
especially  if  the  working  steam 
pressure  is  low.  With  a  boiler 
pressure  of  50  Ibs.  per  square 
inch,  the  temperature  of  the 
injection  water  may  be  as  high 
as  140°  F.  With  a  boiler 
pressure  of  100  Ibs.,  water  at 
a  temperature  of  120°  F.  may 
be  used.  With  a  boiler  pressure 
of  200  Ibs.,  the  temperature  of 
the  injection  water  should  not 
be  more  than  90°,  even  if  fed 
into  the  injector  by  gravity. 

Exhaust  Steam  Injectors. — An  injector  may  be  worked 
by  the  exhaust  steam  from  a  non-condensing  engine ;  such  an 
arrangement  has  the  effect  of  slightly  reducing  the  back  pressure 
in  the  engine  and  of  warming  the  water.  If,  however,  the  load 
on  the  engine  is  very  variable  the  action  of  the  injector  is  rather 
uncertain.  The  objection  to  the  use  of  injectors  is  that  there  is 
a  danger  of  the  passages  becoming  choked  with  sediment,  in 
which  case  the  injector  naturally  ceases  to  work. 


Fig.  23. — Diagram  showing  action 
of  injector. 


STEAM-RAISING   ACCESSORIES. 


63 


Peed-Water  Heaters  and  Economisers. — It  is  uneconomi- 
cal to  supply  a  boiler  with  cold  feed  water,  and  any  heat  which 
would  otherwise  be  wasted  should  be  used  to  heat  it.  When  a 


Fig.  24. — Modern  injector. 

boiler  is  used  Jx>r  driving  a  non-condensing  engine,  the  steam, 
after  it  has   done    its  work   in  the  engine,  can  be  utilised  for 


64 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


warming  the  feed  water  by  means  of  a  simple  apparatus  called  a 
feed-water  heater.     This  consists  of  a  number  of  tubes  placed 

inside  a  shell  or  drum ;  through 
these  tubes,  usually  about  1J 
inches  diameter,  the  water  is 
forced  on  its  way  to  the  boiler, 
the  exhaust  steam  from  the 
engine  is  taken  to  the  shell 
before  passing  away  to  the 
atmosphere ;  it  surrounds  the 
small  tubes  through  which  the 
water  is  passing,  and  gives  up 
a  large  portion  of  its  heat  to 
the  water. 

A  good  feed-water  heater  is 
shown  by  Fig.  25.  It  will  be 
seen  that  the  tubes  are  free  to 
expand  irrespective  of  the 
outer  casing.  Any  impurities 
which  may  be  thrown  down 
from  the  water  by  the  heat  of 
the  exhaust  steam  will  fall  into 
the  deposit  chamber,  from 
whence  the  deposit  can  easily 
be  removed.  The  feed  heater 
shown  is  made  by  Messrs. 
Holden  &  Brooke. 

A  feed-water  heater  of  the 
kind  shown  will  raise  the  tem- 
perature of  the  feed  from  about 
60°  F.  to  about  180°  F.,  when 
worked  at  its  rated  capacity. 
If  worked  below  its  full  capa- 
city, the  water  can  be  made 
still  hotter.  As  will  be  seen 
from  the  illustration  the  feed 
water  passes  once  up  and  once 
down  the  tubes ;  in  the  latest 
heater  made  by  Messrs.  Holden 
&  Brooke  and  called  their  High 
Velocity  feed  heater,  the  water 
is  passed  six  times  up  and  six 
times  down  the  tubes ;  the 


Fig.  25.— Feed-water  heater. 


water  is  also  driven  through  the  tubes  at  a  high  velocity,  the 
result  being  that  the  water  rolling  over  on  itself,  so  to  speak, 


STEAM-RAISING   ACCESSORIES.  65 

in  its  passage  through  the  tubes,  extracts  more  heat  from  the 
steam.  With  this  form  of  heater  the  temperature  of  the  feed 
is  raised  from  60°  to  about  200°  when  worked  at  its  full  rated 
capacity,  but  more  energy  is  expended  in  driving  the  water 
through.  The  saving  effected  through  heating  the  feed  water 
is  approximately  1  per  cent,  of  coal  for  every  10°  by  which  the 
temperature  of  the  feed  is  raised. 

In  cases  where  the  exhaust  steam  is  not  available  for  this 
purpose,  as  in  a  condensing  engine,  the  feed  water  is  often  heated 
by  means  of  an  Economiser.  An  economiser  consists  of  a  large 
number  of  vertical  tubes  about  4  inches  in  internal  diameter  and 
9  feet  long,  placed  in  a  brickwork  setting  at  the  back  end  of  a 
Cornish,  Lancashire,  or  other  boiler;  the  hot  gases  are  compelled 
to  pass  around  these  economiser  tubes  on  their  way  to  the 
chimney ;  the  feed  water  is  forced  through  the  tubes  before 
entering  the  boiler.  It  has  been  found  necessary  to  provide 
means  to  keep  the  tubes  free  from  soot ;  to  effect  this  each  tube 
is  provided  with  a  scraper  which  travels  slowly  up  and  down  its 
exterior;  the  power  to  work  these  scrapers  is  taken  from  the 
engine  by  means  of  shafting,  or  a  small  electric  motor  is  provided 
for  the  purpose. 

If  the  gases  leave  the  boiler  at  a  temperature  of  650°,  an 
economiser  will  extract  about  300°,  reducing  the  temperature  of 
the  gases  to  350°.  The  temperature  of  the  feed  water  passing 
through  the  economiser  will  be  raised  by  about  150°,  say  from 
62°  to  212°,  or  if  the  feed  water  is  taken  from  the  hot  well,  and 
its  temperature  is  already  100°,  the  economiser  will  raise  the 
temperature  to  250°. 

Economisers  are  not  used  so  frequently  with  water-tube  boilers 
as  with  those  of  the  shell  type,  as  the  larger  amount  of  heating 
surface  in  a  boiler  of,  say,  the  Babcock  type,  extracts  a  larger 
percentage  of  the  heat  from  the  gases ;  in  fact,  the  portions  of  the 
tubes  in  a  Babcock  boiler  nearest  the  chimney  may  be  looked 
upon  in  the  light  of  an  economiser,  but  without  the  mechanical 
scrapers.  It  is  not  advisable  to  reduce  the  temperature  of  the 
chimney  gases  below  350°,  if  a  good  draught  is  desired. 

Thermal  Storage. — Another  method  of  heating  the  feed 
water  is  by  means  of  live  steam ;  this  method  is  adopted  in  what 
is  known  as  the  Thermal  Storage  System,  introduced  by  Mr. 
Druitt  Halpin.  The  thermal  storage  system  is  frequently  used 
in  electric  generating  stations  where,  during  the  day  time,  the 
boilers  can  generate  more  steam  than  the  engines  or  turbines 
require,  but  where,  during  the  evening,  when  the  heavy  load 
comes  on,  the  boilers  have  difficulty  in  meeting  the  demand  for 
steam.  The  system  briefly  is  this : — There  is  a  large  drum 

5 


66  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

containing  a  considerable  amount  of  feed  water,  the  drum  being 
cons  ructed  to  bear  the  same  pressure  as  the  boiler.  During  the 
hours  when  the  engines  require  only  a  small  quantity  of  steam,  a 
portion  of  the  steam  generated  is  passed  into  the  thermal  storage 
drum,  and  its  heat  is  given  up  to  the  water.  The  amount  of 
steam  passed  is  such  that  by  the  time  the  evening  comes  on  with 
a  heavy  demand  for  steam,  the  water  in  the  drum  will  have 
received  sufficient  heat  to  enable  it  to  turn  into  steam  without 
the  addition  of  any  further  heat,  should  the  pressure  fall ;  or  the 
addition  of  a  very  small  quantity  of  heat,  obtained  by  passing 
the  water  from  the  storage  drum  through  the  boiler,  will  cause 
the  water  to  evaporate  at  the  full  steam  pressure. 

Superheaters. — The  advantage  of  superheating  steam,  from 
the  point  of  view  of  economy,  has  long  been  known,  but  it  is 
only  within  recent  years  that  superheaters  have  come  into  any- 
thing like  general  use.  By  superheating  steam  before  it  leaves 
the  boiler,  loss  due  to  condensation  of  the  steam  in  the  pipes 
conveying  it  to  the  engine  is  avoided,  and  initial  condensation  in 
the  engine  itself  is  reduced.  As  the  initial  condensation  in  a 
simple  steam  engine  may  be  as  much  as  30  or  40  per  cent., 
the  gain  through  using  superheated  steam  with  such  engines 
is  apparent.  In  compound  or  triple-expansion  engines,  or  in 
turbines,  where  the  initial  condensation  is  less,  the  gain 
through  the  use  of  superheated  steam  is  not  so  great,  but  it 
is  still  considerable. 

In  trials  of  some  high-speed  triple-expansion  engines,  which 
were  carried  out  a  few  years  ago  by  the  makers,  the  saving  due 
to  superheating  the  steam  by  100°  at  the  engine  stop  valve 
varied  from  14  J  to  16|  per  cent,  at  full  load,  and  from  20  to 
22  J  per  cent,  at  one-third  load.  Speaking  generally,  it  may  be 
said  that  with  a  good  compound  or  triple-expansion  engine  a 
saving  of  1  per  cent,  of  steam  at  full  load  is  effected  by  every 
6  per  cent,  of  superheat  given  to  the  steam,  while  in  a  steam 
turbine  a  saving  of  1  per  cent,  is  effected  by  every  10°  to  12°  of 
superheat. 

The  Admiralty  recently  carried  out  some  trials  to  ascertain 
the  value  of  superheated  steam  in  H.M.S.  Britannia,  and  it  was 
found  that  at  cruising  speed,  when  the  engines  developed  one- 
fifth  of  their  power,  the  saving  effected  through  superheating  the 
steam  by  83°  at  the  engines  was  15  per  cent. 

A  superheater  consists  of  a  number  of  small  tubes,  which  are 
placed  in  the  path  of  the  furnace  gases,  and  through  which  the 
steam  is  passed.  In  Cornish  and  Lancashire  boilers  the  super- 
heater is  usually  placed  in  the  flue  at  the  end  of  the  boiler; 
dampers  are  provided  to  prevent  the  steam  becoming  too  highly 


STEAM-RAISING  ACCESSORIES.  67 

superheated.  If  the  steam  is  too  highly  superheated,  it  car- 
bonises the  oil  used  in  the  engine  cylinder  and  valve  chests, 
as  the  best  mineral  oil  will  not  stand  a  temperature  higher  than 
650°,  and  few  oils  will  bear  a  temperature  higher  than  600°. 

For  reciprocating  engines  the  amount  of  superheat  usually 
employed  is  about  100°,  and  occasionally  150°  •  thus  with  steam 
at  a  pressure  of  180  Ibs.  the  total  temperature  is  about  480°,  and 
occasionally  530°.  In  a  steam  turbine  the  degree  of  superheat 
is  limited  by  the  temperature  which  the  bronze  blades  will  safely 
bear. 

In  the  Babcock  boiler  the  superheater,  as  will  be  seen  by  Fig. 
20,  is  above  the  main  steam-raising  tubes  and  below  the  drum ; 
in  this  position  it  is  out  of  the  way  of  the  fierce  heat  of  the 
furnace,  and  special  dampers  are  not  required.  The  small  pipe 
leading  from  the  drum  to  the  superheater  is  for  flooding  the 
latter  while  steam  is  being  raised. 

In  the  M'Phail  &  Simpson  superheater,  which  was  probably 
the  first  really  successful  superheater  used  in  this  country,  the 
steam,  after  being  superheated  in  pipes  placed  in  the  flues  of  the 
boiler,  was  taken  through  a  pipe  which  passed  through  the  water 
space  of  the  boiler ;  the  effect  of  this  was  to  take  some  of  the 
sting  out  of  the  steam,  or,  in  other  words,  to  prevent  an  excessive 
degree  of  superheat  being  reached. 

In  the  Cruse  superheater  the  tubes  are  6  inches  diameter,  and 
have  an  internal  pipe  made  of  copper  through  which  a  stream  of 
water  is  passed.  This  water  may  be  drawn  from  the  water  space 
of  the  boiler,  from  the  economiser,  or  from  the  cold  water  feed 
mains,  and  in  this  way  the  amount  of  superheat  can  be  regulated. 

Some  superheaters  are  separately  fired.  The  best  known 
superheater  of  this  type  is,  perhaps,  the  Schmidt,  in  which  there 
are  two  sets  of  coils,  an  upper  and  lower.  The  saturated  steam 
enters  at  the  top — i.e.,  farthest  from  the  fire — and  travels  down 
through  the  coils ;  dampers  are  provided,  by  means  of  which  the 
degree  of  superheat  can  be  regulated.  A  separately-fired  super- 
heater, although  conducing  to  economy,  can  hardly  effect  so  great 
a  saving  as  one  making  use  of  gases  which  otherwise  would  be 
wasted.  A  Schmidt  superheater  using  waste  gases  is  also  made. 

Mechanical  Stokers. — In  cases  where  several  boilers  are 
installed,  or  where  there  is  a  single  boiler  the  output  from  which 
is  fairly  constant,  it  is  an  advantage  to  fit  each  boiler  with  a 
mechanical  stoker.  This  apparatus  enables  the  number  of  men 
stokers  in  a  large  power-house  station  to  be  reduced ;  but  what 
is  of  much  greater  importance  is  the  fact  that  a  good  mechanical 
stoker  allows  a  very  cheap  quality  of  coal  to  be  used  and  burnt 
smokelessly.  Small  coal  known  as  slack,  which,  without  a 


68  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

mechanical  stoker  is  comparatively  useless  for  raising  steam,  is 
now  very  largely  used  ;  it  contains  from  10,000  to  12,000  B.T.U. 
per  lb.,  and  can  be  obtained  at  prices  varying  from  three  shillings 
and  sixpence  to  ten  shillings  per  ton,  the  price  varying  with  the 
locality  and  with  the  demand,  or  otherwise,  for  this  very  small 
coal.  A  mechanical  stoker  adds  a  small  quantity  of  coal  to  the 
fire  at  regular  intervals,  and  thus  ensures  more  perfect  com- 
bustion than  is  possible  when  a  mass  of  coal  is  thrown  on  at  a 
time. 

There  are  various  kinds  of  mechanical  stokers ;  the  difference 
between  each  consists  chiefly  in  the  manner  in  which  the  coal  is 
put  on  to  the  fire.  In  one,  the  Bennis,  the  coal  is  thrown  on  in 
the  form  of  spray ;  this  is  called  a  sprinkling  stoker.  In  another, 
the  Vicars,  the  small  coal  is  pushed  forward  by  means  of 
plungers;  this  form  of  stoker  is  called  a  coking  stoker,  as  the 
small  coal  gets  coked  at  a  very  early 
stage  in  the  process  of  combustion. 

Another  form  is  the  underfeed  stoker. 
In  this  stoker  the  centre  of  the  fire 
grate  is  higher  than  the  sides ;  there 
is  an  opening  running  down  the  whole 
length  of  the  grate  through  which  coal 
is  fed  up  from  below  by  means  of  a 
worm ;  the  bars  fall  away  on  each  side 
of  the  opening  and  air  is  driven  in 
between  them.  Fig.  26  shows  a  section 
Fig.  26.—"  Underfeed  "  through  the  furnace  and  bars. 

stoker.  In  this  form  of  stoker  the  coal  gets 

coked  early  and  very  perfect  combustion 

is  obtained.  In  most  stokers  of  the  above  types  there  is  a 
device  for  rocking  the  fire  bars,  so  that  the  burning  fuel  i& 
gradually  carried  forward  until  it  reaches  the  end  of  the  grate, 
when  it  falls  over  in  the  form  of  clinker  and  ash.  In  the 
Bennis  stoker  a  small  blast  of  steam  is  employed  to  force  air 
into  the  furnace.  Air  for  the  underfeed  stoker  is  driven  in 
by  a  fan. 

Perhaps  the  best  and  simplest  form  of  mechanical  stoker  is- 
that  of  the  chain  grate  type.  In  this  stoker  the  fire  bars  are 
connected  together  so  as  to  form  an  endless  chain,  which  is 
moved  slowly  over  drums,  thus  carrying  the  fuel  forward  in  a 
very  regular  and  even  manner.  An  advantage  which  this  stoker 
has  over  many  others  is  that  the  grate  may  be  of  unlimited 
length.  Until  recently  the  very  smallest  form  of  slack  could  not 
be  used  with  chain  grate  stokers,  but  the  introduction  of  a  new 
form  of  fire  bar  has  overcome  this  difficulty. 


STEAM-RAISING  ACCESSORIES.  69 

An  objection  which  is  sometimes  urged  against  a  mechanical 
stoker  is  that  it  is  somewhat  difficult  to  force  a  boiler  so  fitted, 
should  an  exceptional  quantity  of  steam  be  required  in  an 
emergency.  If,  however,  the  stoker  is  combined  with  some 
system  of  blast  or  forced  draught,  or  if  the  speed  of  the  chain 
grate  can  be  accelerated  or  retarded,  the  objection  does  not  hold 
good. 

Although  not  necessarily  connected  with  mechanical  stokers, 
two  well-known  systems  of  increasing  the  draught  in  a  furnace 
may  be  mentioned.  They  are  the  Howden  system  (as  used 
chiefly  in  marine  work)  and  the  Meldrum  blower  system.  In 
the  Howden  system  the  air  is  heated  by  being  passed  through 
pipes  placed  in  the  uptake  from  the  boiler;  the  heated  air  is 
then  supplied  under  pressure  both  above  and  below  the  firegrate. 
In  the  Meldrum  system  steam  is  superheated  in  a  pipe  placed  in 
the  furnace,  and  is  then  discharged  through  a  trumpet-shaped 
blower  below  the  furnace ;  by  this  system  very  small  coal  can  be 
burnt  smokelessly  and  without  a  mechanical  stoker. 

Coal  Conveying  Plant. — The  large  number  of  boilers  in  a 
power-generating  station,  and  the  use  of  mechanical  stokers, 
render  it  almost  essential  that  the  coal  should  be  conveyed  to 
the  hoppers  of  the  stokers  by  mechanical  means.  This  is  usually 
effected  by  means  of  elevators,  conveyers,  and  occasionally  by  a 
crane  and  grab.  The  coal  bunkers,  as  a  rule,  are  placed  above 
the  boilers,  and  shoots  convey  the  coal  from  these  bunkers  to 
the  hoppers  of  the  stokers. 

An  elevator  consists  of  a  series  of  pressed  steel  buckets,  about 
12  inches  wide,  which  are  attached  to  chains  passing  over  drums; 
as  the  buckets  travel  round  the  lower  drum  they  come  in  contact 
with  the  coal,  either  in  a  barge,  bunker,  or  other  receptacle,  and 
fill  themselves  ;  when  the  buckets  reach  the  top  of  the  elevator 
they  tip  over  and  discharge  their  contents  into  a  shoot  leading 
to  a  conveyer. 

A  conveyer  may  be  either  of  the  bucket  or  chain  type ;  in  the 
latter  the  conveyer  is  merely  a  steel  trough,  usually  from  9 
to  18  inches  wide,  and  from  6  to  9  inches  deep.  At  the 
bottom  of  this  trough  a  flat  chain  resembling  a  ladder  moves 
slowly  along;  the  links  of  the  chain  are  about  2  or  2J 
inches  deep,  about  J  inch  thick,  and  spaced  about  12  inches 
apart.  As  the  chain  moves  along  it  drags  the  coal  with  it ;  the 
trough  passes  over  shoots  which  lead  to  hoppers  on  the  boilers, 
and  in  the  trough,  directly  over  each  shoot,  is  an  opening  with  a 
sliding  door ;  a  certain  proportion  of  the  coal,  as  it  travels  along 
the  trough,  falls  through  these  openings,  the  exact  amount  being 
regulated  by  opening  or  closing  the  sliding  doors.  These 


70  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

conveyers  will  convey  coal  up  an  incline  of  30°  or  at  an  even 
greater  angle,  so  that  in  cases  where  the  coal  wharf  or  siding  is 
at  some  distance  from  the  boiler-house  the  coal  may  be  conveyed 
up  a  gradual  incline  to  the  bunkers  on  the  first  floor,  and  an 
elevator  may  be  dispensed  with. 

If  it  is  desired  to  convey  the  coal  at  right  angles  to  the  main 
conveyer,  the  coal  is  allowed  to  drop  through  an  opening  in  the 
trough  and  fall  on  to  a  second  conveyer.  These  conveyers  are 
driven  either  by  motors,  by  shafting  from  the  engine-house,  or 
by  a  separate  engine. 

The  bucket  form  of  conveyer  consists  of  a  series  of  small 
buckets  attached  to  two  double  flat-link  chains  ;  the  chains  are 
provided  with  wheels  which  run  on  suitable  rails.  When  it  is 
desired  to  discharge  coal  from  this  form  of  conveyer,  the  bucket 
is  tipped  up  by  means  of  a  cam.  Special  fillers  are  provided  to 
ensure  that  the  coal  falls  into  the  buckets  only  as  they  pass.  A 
conveyer  of  the  bucket  type  requires  less  power  to  drive  it  than 
one  of  the  plain  chain  and  trough  type. 

Oil  Filters. — When  the  steam  from  an  engine  is  condensed 
and  is  used  over  and  over  again  for  feeding  the  boiler,  it  is 
necessary  to  remove  the  oil  from  it.  Oil  filters  are  of  two 
different  kinds,  one  designed  to  extract  the  oil  from  the  steam 
before  it  is  condensed,  the  other  to  remove  the  oil  from  the 
condensed  steam  or  water.  Of  the  former  type  the  best  known 
examples  are  the  Baker  and  the  Templer-Ranoe.  The  principles 
upon  which  these  work  are  similar;  the  steam  is  expanded  into 
a  large  chamber,  when  some  of  the  heavier  loose  particles  of  oil 
are  thrown  down  by  gravity.  The  steam  then  strikes  against 
baffle  plates,  and  its  direction  is  altered,  when  more  oil  is 
thrown  down.  In  the  Templar-Ranoe  separator  the  baffle  plates 
are  hollow  with  small  lip-like  openings,  the  oil  as  it  trickles 
down  these  baffles  enters  the  openings  and  is  not  liable  to  be 
again  licked  up  by  the  steam.  Such  mechanical  oil  filters  will 
extract  about  98  per  cent,  of  the  oil  from  the  steam.  In  the 
majority  of  cases  the  remaining  2  per  cent,  is  harmless,  in  others 
trouble  arises  from  it.  One  of  the  most  effective  filters  for 
extracting  the  oil  from  steam  after  condensation  consists  of 
chambers  containing  sawdust,  through  which  the  condensed 
steam  is  passed.  The  sawdust,  however,  requires  to  be  removed 
and  replaced  at  somewhat  frequent  intervals.  The  only  way  to 
remove  the  whole  of  the  oil  from  steam  is  to  treat  it  chemically 
after  condensation.  Plants  for  this  "purpose  are,  however, 
somewhat  expensive  and  occupy  a  good  deal  of  space. 


71 


CHAPTER  Y. 
STEAM  PIPES  AND  VALVES. 

Steam  Pipes. — The  pipes  conveying  steam  from  the  boilers 
to  the  engines  or  turbines  are  usually  made  of  wrought  iron  or 
mild  steel,  and  are  lapwelded,  solid-drawn,  or  riveted.  Some 
pipe  makers  still  prefer  wrought  iron  to  steel  for  making  lap- 
welded  pipes,  as  in  their  opinion  a  more  reliable  weld  can  be 
obtained  with  the  former.  Solid  drawn  steel  tubes  can  be 
obtained  up  to  10  inches  diameter,  but  they  are  expensive. 
Lapwelded  tubes  are  made  up  to  14  inches  diameter.  Pipes 
above  14  inches  diameter  are  usually  made  of  riveted  mild  steel. 
Cast-iron  pipes  should  not  be  used  for  steam  pressures  above 
80  Ibs.  per  square  inch. 

The  flanges  at  the  ends  of  wrought-iron  and  steel  pipes  are 
made  of  mild  steel,  wrought  iron,  cast  iron,  or  cast  steel.  They 
are  usually  screwed  on  to  the  pipe,  the  end  of  the  latter  being 
expanded  and  riveted  over  into  a  space  or  recess  left  in  the 
flange  for  the  purpose.  Wrought-iron  flanges  are  sometimes 
welded  on,  but  unless  the  welding  is  done  thoroughly,  a  screwed 
and  riveted  flange  is  to  be  preferred.  The  flanges  of  riveted  pipes 
of  large  size  are,  of  course,  riveted  on.  The  joint  between  the 
flanges,  which  are  faced,  consists  usually  of  a  soft  copper  ring  or 
of  a  brass  corrugated  ring.  Any  jointing  material,  such  as 
asbestos,  which  may  blow  out,  should  be  avoided. 

Arrangement  of  Pipes. — In  laying  out  a  pipe  arrangement, 
care  should  be  taken  to  avoid  hollows  or  pockets  where  water 
can  collect.  In  cases  where  a  pocket  is  unavoidable,  it  should 
be  connected  by  a  drain  pipe  to  a  steam  trap.  These  will  be 
described  later. 

Most  engineers  are  fully  alive  to  the  danger  resulting  from 
water  lying  in  pipes,  should  it  be  necessary  for  these  to  dip 
down  from  the  boiler  and  rise  again  to  the  engines,  and  great 
care  is  usually  taken  to  see  that  such  pipes  are  free  from  water 
before  starting  the  engines,  and  to  keep  them  well  drained  while 
the  engines  are  running;  but  there  is  another  arrangement 
frequently  met  with,  which  is  almost  as  dangerous,  and  which 
is  probably  responsible  for  a  large  proportion  of  the  accidents 
occurring  through  the  presence  of  water  in  steam  pipes.  The 
arrangement  referred  to  consists  of  a  long  horizontal  or  slightly 


72  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

sloping  length  of  pipe  with  a  vertical  rise  at  one  end,  beyond 
which  the  engines  take  their  steam.  The  arrangement  is  shown 
by  Fig.  27. 

The  sketch  shows  a  long  pipe  conveying  steam  to  engines 
A  and  B.  Any  water  which  condenses  in  this  pipe  is  carried 
forward  by  the  passage  of  the  steam  to  the  bend  C,  where  it 
collects  and  obstructs  the  pipe  to  the  extent  of  perhaps  one-half 
of  its  area  or  even  more ;  this  obstruction  may  not  be  serious  so 
long  as  the  engine  supplied  from  flange  A,  only,  is  working,  but 
should  the  attendant  suddenly  start  up  the  engine  connected  to 
flange  B,  the  sudden  rush  of  steam  past  the  bend  C  will  probably 
pick  up  the  water  lodged  there,  and  carry  it  forward  like  a 
bullet,  possibly  wrecking  the  engine  at  B  or  some  other  engine 
further  along  the  line  of  piping.  If  such  a  vertical  rise  after  a 


C 
Fig  27. — Dangerous  pipe  arrangement. 

long  length  of  horizontal  piping  is  necessary,  the  bend  should 
have  a  large  pocket  with  a  drain  pipe  leading  from  it  to  a  steam 
trap. 

One  occasionally,  in  past  years,  received  advice  to  arrange 
steam  pipes  with  a  gradual  slope  down  towards  the  boiler,  so 
that  all  the  water  in  the  pipes  might  drain  back  to  it.  This 
advice  is  radically  bad,  as  a  moment's  consideration  will  show, 
for  with  the  pipes  full  of  steam  under  pressure,  and  the  steam 
travelling  away  from  the  boiler  at  a  rate  of  perhaps  6,000  feet 
per  minute,  it  is  practically  impossible  for  any  water  to  travel 
back  against  the  current  of  steam.  The  plan  is  bad,  for  another 
reason,  viz.  : — When  the  engine  is  not  working,  and  the  boiler 
stop  valve  is  closed,  any  steam  condensed  in  the  pipe  will  flow 
back  towards  the  boiler,  and  will  lie  either  on  the  stop  valve,  or 
in  the  pipes  adjacent  to  it.  When  the  boiler  stop  valve  is  next 
opened,  the  water  collected  upon  it,  unless  previously  drained 
off,  will  have  to  be  got  rid  of  somehow. 

The  stop  valve  on  the  boiler  should  be  arranged  as  shown  by 
Fig.  28,  and  not  as  shown  by  Fig.  29. 

The  right  way  to  arrange  steam  pipes  is  to  erect  them  with  a 
slight  fall  all  the  way  from  the  boilers  to  the  engines,  and  to 
provide  each  engine  with  a  steam  separator  or  dryer.  This 


STEAM  PIPES  AND  VALVES. 


73 


dryer  may  be  merely  a  receptacle  arranged,  so  as  to  change  the 
direction  of  the  flow  of  steam,  and  to  hold  the  water  as  caught. 
If  the  direction  in  which  steam  is  travelling  at  a  high  velocity 
is  suddenly  altered,  any  large  particles  of  water  entrained  in  it 
will  be  thrown  down.  The  steam  dryer  should  have  a  gauge 
glass  to  show  the  amount  of  water  lying  in  it,  and  be  provided 
with  a  drain  cock  connected  either  to  a  trap,  or  to  a  pipe,  lead- 
ing to  a  sump  or  hot  well.  Any  valves  on  branch  pipes  leading 
out  of  the  main  steam  pipe  should  be  placed  close  up  to  the 
latter,  so  that  water  may  nob  collect  in  the  branches  above  the 
valves  when  the  latter  are  closed. 

In  designing  pipe  work,  another  point  to  be  borne  in  mind  is 
the  expansion  and  contraction  of  the  pipes  due  to  the  difference 
of  temperature  when  full  of  steam,  and  when  empty  and  cold. 
Wrought  iron  expands  -0000067  times  its  own  length  for  every 
degree  Fahr.  in- 
crease of  tempera- 
ture between  32° 
and  212°,  and  as 
much  as  -0000089 
times  its  own 
length  for  every 
degree  Fahr.  when 
the  range  of  tem- 
perature is  between 
32°  and  500°,  so 
that  if  we  have  a 
pipe  50  feet  long 


Fig.  28.  Fig.  29. 

Positions  of  stop  valve  on  boiler. 


600  inches,  and  its  temperature  rises  from  50°  to  480°  (which 
temperature  is  reached  with  steam  at  185  Ibs.  pressure  and  100° 
of  superheat),  we  have  a  difference  of  430°.  The  expansion, 
therefore,  is  600  x  430  x  -0000089  =  2-29  inches,  a  total  ex- 
pansion of  2-29  inches  in  50  feet  of  pipes. 

It  is  usual  to  provide  for  such  expansion  by  having  easy 
wrought-iron  or  mild  steel  bends  at  each  end  of  the  pipes, 
and  easy  bends  where  the  pipes  join  and  leave  the  main  steam 
range.  Formerly  copper  bends  were  used  to  take  up  the  ex- 
pansion, but  as  copper  rapidly  loses  its  strength  under  high 
temperature,  the  use  of  such  copper  bends  has  been  practically 
abandoned. 

Where  it  is  impossible  to  provide  bends  to  take  up  the 
expansion,  an  expansion  joint,  consisting  of  one  pipe  running 
into  another  of  larger  diameter,  and  provided  with  a  gland,  is 
employed ;  the  gland  is  packed  with  asbestos,  or  other  packing, 
in  the  same  way  as  the  gland  of  a  piston-rod  is  packed. 


74  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

In  fixing  steam  pipes  they  should  be  supported  on  rollers,  or 
slung  in  such  a  way  that  the  contraction  and  expansion  of  the 
pipes  may  not  strain  any  of  the  joints. 

When  electric-generating  stations  were  first  installed  in  this 
country,  it  was  customary  for  some  engineers  to  employ  dupli- 
cate steam  mains,  in  order  to  guard  against  any  interruption  to 
the  supply  of  current,  in  the  event  of  the  failure  of  a  pipe  or 
pipe  joint.  This  arrangement  necessitated  a  very  large  number 
of  valves,  which  were  in  themselves  a  source  of  weakness ;  and 
in  any  duplicate  system  there  is  one  point  where  the  two  pipes 
converge  into  a  Y-piece,  usually  on  the  separator  of  the  engine. 
Should  the  joint  of  this  Y-piece  blow  out,  the  elaborate  duplicate 
system  is  rendered  useless.  It  is  now  more  usual  to  have  one 
large  pipe  into  which  the  boilers  feed,  and  from  which  the 
engines  or  turbines  take  their  supply.  This  pipe  may  be  divided 
into  sections  by  means  of  full-way  valves,  if  desired. 

Size  of  Pipes. — The  size  of  steam  pipes  should  be  such  that 
the  flow  of  steam  in  them  does  not  exceed  a  speed  of  from  5,000 
to  6,000  feet  per  minute  (5,000  feet  is  preferable  for  pipes  of 
4  inches  and  less  in  diameter) ;  and  if  the  amount  of  steam  to  be 
carried  is  known,  the  area  of  the  pipe  can  easily  be  calculated 
on  this  basis  by  simple  arithmetic.  The  quickest  way  to  make 
the  calculation  is  by  the  formula  given  below.  It  should  be 
remembered  that  the  greater  the  speed  of  the  steam  in  the  pipes 
the  greater  is  the  drop  in  pressure  at  the  far  end  of  the  pipe, 
or,  expressed  in  another  way,  a  greater  amount  of  pressure  is 
required  to  force  steam  through  pipes  at  a  high  speed  than  at  a 
low  one.  (The  resistance  varies  with  the  square  of  the  velocity.) 
If  the  boiler  is  capable  of  producing  steam  at  a  pressure  higher 
than  is  required  by  the  engine,  .then  there  is  not  the  same 
objection  to  employing  small  pipes ;  but  if  the  engine,  in  order 
to  develop  its  full  power,  requires  steam  at  approximately  the 
same  pressure  as  that  generated  in  the  boiler,  then  pipes  of 
ample  size  should  be  provided. 

The  formula  for  ascertaining  the  area  of  a  pipe  to  convey  a 
certain  number  of  pounds  of  steam  at  a  given  speed  is  as 
follows  : — 

P  x  Y  x  144 

—      -  =  A; 

where     P  =  pounds  of  steam  per  minute. 

Y  =  volume  in  cubic  feet  of    1   Ib.   of  steam  at   the 

pressure  to  be  employed  (given  in  Table  ix.) 
S  =  speed  permitted  in  feet  per  minute. 
A  =  area  of  pipe  in  square  inches. 


STEAM  PIPES  AND  VALVES. 


75 


Example. — What  size  of  pipe  is  required  to  convey  steam  to  a  turbine 
using  54,000  Ibs  per  hour,  or  900  Ibs.  per  minute,  assuming  the  rate  of 
flow  in  the  pipe  is  not  to  exceed  6,000  feet  per  minute,  the  steam  pressure 
being  150  Ibs.  ?  The  calculation  is  — 

900  x  2-71  x  144  _ 
~  6,000 

From  a  table  of  areas,  it  will  be  seen  that  a  pipe  8§  inches  diameter  has 
an  area  of  58  '42,  so  that  a  pipe  of  this  size  would  suffice  ;  but  wrought  iron 
and  steel  pipes  are  not  made  in  odd  sizes,  so  that  a-pipe  of  the  nearest  even 
size  —viz. ,  9  inches — would  be  selected. 

If  the  area  of  a  pipe  is  known,  and  it  is  desired  to  ascertain 
how  many  cubic  feet  of  steam  will  flow  through  it  per  minute 
at  a  given  linear  speed,  the  calculation  is,  of  course,  as  follows : — 

A  x  S 
....      =  cubic  feet. 

144: 

Example. — How  many  cubic  feet  of  steam  will  flow  through  a  pipe 
6  inches  diameter  (the  area  of  which  is  28 '27  square  inches),  if  the  speed 
of  the  steam  is  not  to  exceed  6,000  feet  per  minute  ? 

28-27  x  6,000 

y^r =1,177  cubic  feet. 

If  the  number  of  cubic  feet  of  steam  is  known,  it  can  be 
converted  into  pounds  of  steam  by  the  aid  of  Table  ix. 

The  following  table,  giving  the  amount  of  steam  in  cubic  feet 
per  minute,  and  in  pounds  per  hour,  which  will  pass  through 
pipes  of  various  sizes  when  the  speed  is  limited  to  6,000  linear 
feet,  may  be  useful.  The  approximate  loss  of  pressure  for  every 
100  feet  of  straight  pipe  is  also  given  : — 

TABLE  XI.— KATE  OF  FLOW,  6,000  FEET  PER  MINUTE. 


Steam  at  150  Ibs.  (gauge) 

Steam  at  200  Ibs.  (gauge) 

pressure. 

pressure. 

Diameter 
of  pipe. 

Cubic  feet 
per  minute. 

Lbs.  pei- 

Approximate 
drop  of 

Lbs.  per 

Approximate 
drop  of 

hour. 

pressure  per 

hour. 

pressure  per 

100  feet. 

100  feet. 

Ins. 

Lbs.  per  sq.  in. 

Lbs.  per  sq.in. 

3 

294 

6,514 

4-0 

8,320 

5-25 

4 

523 

11,579 

3-0 

14,802 

4-0 

6 

1,177 

26,059 

2-0 

33,311 

2-6 

9 

2,650 

58,671 

1-4 

75,000 

1-75 

12 

4,708 

104,236 

1-0 

133,245 

1-3 

18 

10,600 

234,686 

•6 

300,000 

•87 

76  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

The  number  of  cubic  feet  of  steam  per  minute  that  will  flow 
through  a  pipe  of  a  given  size  and  at  a  given  speed  in  linear 
feet  per  minute  is,  of  course,  the  same,  whatever  the  working 
pressure  of  the  steam  may  be;  but  as  the  density  of  steam  is 
greater  at  high  than  at  low  pressures,  a  larger  number  of  Ibs.  of 
high-pressure  steam  will  come  through  than  would  be  the  case 
with  low-pressure  steam  ;  the  pressure  or  head  required  to  drive 
the  denser  steam  through  is,  however,  greater. 

Thus,  if  we  take  the  case  of  a  6-inch  pipe,  in  which  the  steam 
flows  at  a  rate  of  6,000  feet  per  minute,  26,059  Ibs.  of  steam  at 
150  Ibs.  pressure  will  be  delivered  with  a  fall  of  pressure  of 
about  2  Ibs.  in  each  100  feet  of  straight  length  of  pipe ;  but  with 
steam  at  200  Ibs.  pressure,  33,311  Ibs.  will  come  through,  but 
the  fall  of  pressure  will  be  about  2-6  Ibs.  per  100  feet,  the 
number  of  cubic  feet  coming  through  being  1,777  in  each  case. 

The  generally-accepted  formula  dealing  with  the  flow  of  steam 
in  pipes  is  given  below,*  but  this  formula  is  interesting  as 
showing  what  the  steam  should  do  in  smooth  pipes,  rather  than 
what  it  actually  does  in  practice. 

In  actual  working  practice  there  are  many  factors  which  may 
entirely  vitiate  the  results  of  calculations  based  on  elaborate 
formulae  which  cannot  well  take  these  factors  into  account.  For 
instance,  in  a  long  range  of  straight  steam  pipe  there  is,  at  a 
distance  of  every  12  feet  or  so,  a  tiny  groove  formed  by  the 
space  required  for  the  packing  placed  between  the  flanges.  This 
packing  ring  cannot  be  made  of  the  same  internal  diameter  as 
the  pipe,  on  account  of  the  danger  of  a  part  of  the  packing 
projecting  into  the  bore  of  the  pipe  and  thus  causing  an  obstruc- 
tion. These  tiny  grooves  cause  eddies  in  the  steam  and  tend 
to  impede  its  flow.  Again,  in  erecting  the  pipes  one  cannot  be 
sure  that  the  centre  of  each  length  of  pipe  will  coincide 
absolutely  with  that  of  its  neighbour.  There  is  a  certain  amount 
of  clearance  in  the  flange  bolt  holes,  and  it  is  hardly  probable 
that  all  the  pipes  in  a  long  range  are  absolutely  true  one  with 
another.  The  wetness  or  dryness,  too,  of  the  steam  affects  its 
rate  of  flow. 

The  obstruction  to  the  flow  of  steam  caused  by  a  globe  valve 


W  = 


W  =  weight  in  Ibs.  per  minute. 

D  =  weight  per  cubic  foot  of  steam  at  the  working  pressure. 
p1  and  />2  =  the  initial  and  final  pressures. 
L  =  length  of  pipe  in  feet. 
d  =  diameter  of  pipe  in  inches. 


STEAM  PIPES  AND  VALVES.  77 

is  sometimes  stated  to  be  equivalent  to  so  many  feet  of  straight 
pipe,  varying  from  50  feet  in  a  3-inch  pipe  to  80  feet  in  a  1 2-inch 
pipe,  but  if  the  globe  valve  is  not  drained,  and  water  accumulates 
in  it  as  shown  by  Fig.  30,  no  formula  will  indicate  what 
obstruction  is  caused. 

Even  if  properly  drained,  the  roughness  and  shape  of  the  valve 
body  will  affect  the  rate  of  flow  considerably. 

A  method  sometimes  adopted  by  a  draughtsman  to  settle  the 
size  of  steam  pipes,  is  to  ascertain  the  diameter  of  the  inlets 
upon  all  the  engines  to  be  supplied  with  steam,  and  which  will 
be  working  at  the  same  time,  and  then  to  arrange  for  his  steam 
pipe  to  have  a  corresponding  area.  For  instance,  should  there 


Fig.  30.—  Stop  valve  passage  obstructed  by  water. 

be  two  engines  each  with  an  inlet  of  6  inches  diameter,  and  one 
with  an  inlet  of  5  inches  diameter,  the  total  area  of  these  inlets 
amounts  to  76-  1  square  inches;  the  draughtsman  turns  to  a  table 
of  areas  and  finds  that  a  pipe  10  inches  diameter  has  an 
area  of  78'5  square  inches;  he  then  settles  on  this  size  of 
pipe.  If  there  are  a  large  number  of  engines  this  method 
gives  a  size  of  pipe  larger  than  is  really  needed,  as  the  engine 
maker,  in  settling  the  size  of  his  inlet,  usually  allows  a  little 
margin. 

The  principal  objection  to  the  use  of  pipes  of  unduly  large 
size,  is  the  heavy  first  cost,  both  of  pipes  and  valves.     There 

A  good  empirical  formula  for  giving  the  flow  of  steam  in  pipes  is  as 
follows  :  — 


g  = 


x  F  x 


where  S  =  speed  (in  linear  feet)  of  the  steam  per  second. 

V  =  volume  in  cubic  feet  of  1  Ib.  of  steam  at  the  working  pressure. 

F  =  fall  in  pressure. 

D  =  diameter  of  the  pipe  in  feet. 

L  =  length  of  the  pipe  in  feet. 


78  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

is  also  a  greater  loss  from  radiation  with  a  large  pipe  than 
with  a  small  one.  It  may  be  mentioned  that  the  loss  from 
this  source  is  about  13  or  14  B.T.U.  per  square  foot  of  an 
uncovered  pipe.  This  loss  may  be  reduced  to  one  varying  from 
1J  to  4  B.T.U.  per  square  foot  if  the  pipe  is  covered  with  one 
of  the  various  non-conducting  compositions  sold  for  the  purpose. 

On  the  other  hand,  a  large  steam  pipe  provides  a  certain 
reservoir  of  steam ;  this  is  an  advantage  when  the  steam  is 
required  for  a  slow-speed  reciprocating  engine,  which  demands  a 
large  supply  of  steam  while  the  admission  port  is  open,  and 
requires  no  steam  while  the  port  is  closed.  This  matter  will  be 
referred  to  again  under  valves. 

The  effect  of  superheating  steam  is  to  increase  its  volume,  not 
its  pressure,  but  superheated  steam  travels  along  a  pipe  with  less 
friction  than  wet  steam  and  prevents  any  obstruction  due  to 
water,  such  as  is  shown  by  Fig.  30. 

The  increase  of  volume  caused  by  superheating  steam  is 
approximately  as  follows  : — 


100°  Superheat. 
Increase  of  volume. 

150°  Superheat. 
Increase  of  volume. 

100  Ibs.  pressure,  . 

150   „           „        ,         ,         . 

200  „           „        . 

'12  5  per  cent. 
12-0       ,, 
11-8       „ 

19  per  cent. 
18        „ 

17        „ 

If,  therefore,  pipes  are  designed  so  that  a  certain  weight  of 
saturated  steam,  at  200  Ibs.  pressure,  should  flow  at  6,000  feet 
per  minute,  then  the  same  weight  of  steam  superheated  by  100° 
will  require  to  flow  at  about  6,708  feet  per  minute.  There 
would  be  no  real  objection  to  this  increased  rate  of  flow,  as 
superheated  steam  approaches  more  nearly  to  a  perfect  gas  than 
saturated  steam,  and  its  friction  is  less. 

Strength  of  Pipes. — The  tensile  stress  in  the  material  of  a 
pipe  or  cylinder  under  internal  pressure  is  found  by  the  follow- 
ing formula: — 

S_P  xP. 

23T3F; 

where  S  =  Tensile  stress  per  square  inch. 
D  =  Diameter  of  pipe  in  inches. 
P  =  Pressure  in  pounds  per  square  inch. 
T  =  Thickness  of  pipe  in  inches,  or  parts  of  an  inch. 

In  the  case  of  lapwelded  wrought-iron  and  steel  pipes,  it  is 
not  customary  to  allow  a  stress  equal  to  the  safe  working  stress 


STEAM  PIPES  AND  VALVES. 


79 


of  the  metal,  as  the  strength  of  the  weld  and  the  stiffness  of  the 
pipe  generally  have  to  be  taken  into  account. 

The  usual  thickness  of  wrought-iron  and  steel  pipes  suitable 
for  working  up  to  pressures  of  200  Ibs.  per  square  inch  is  as 
follows : — 

TABLE  XII. 


Lapwelded. 

Diameter 

Solid  Drawn 

of  Pipe. 

Steel. 

Wrought  Iron. 

Steel. 

Inches. 

Inches. 

Inches. 

3 

i 

i 

4 

i 

I 

5 

About  j1,  less 

6 

5 

TIT 

* 

in 

7 

A 

thickness 

8 

A 

A 

than 

9 

A 

lapwelded 

10 

H 

A 

steel 

12 

1 

1 

tubes. 

14 

A 

1 

If  the  reader  will  apply  the  formula  given  above  to  the  pipes 
in  the  table,  he  will  find  that  with  a  working  pressure  of  200  Ibs. 
the  stress  in  the  metal  is  only  about  J  ton  per  square  inch  in  the 
case  of  the  3-inch  pipe ;  below  1  ton  in  the  case  of  the  4-inch, 
5-inch,  and  6-inch  pipes  ;  and  less  than  1 J  tons  in  the  case  of  the 
12-inch  pipe. 

As  in  practice  the  metal  is  stressed  to  such  a  small  extent 
there  is  but  little  advantage  in  employing  mild  steel  for  lap- 
welded  pipes  if  a  better  weld  can  be  obtained,  which  is  doubtful, 
with  wrought  iron. 

Lapwelded  pipes  of  less  than  3  inches  diameter,  whether  for 
steam  or  gas,  are  made  of  a  uniform  outside  diameter — this 
diameter  will  be  found  in  the  table  of  gas  threads  (Table  v., 
Chap,  ii.)— and  the  inside  diameter  is  made  greater  or  smaller 
according  to  the  pressure  the  pipe  is  required  to  withstand. 
Thus  a  2-inch  steam  pipe  is  of  a  slightly  smaller  internal 
diameter  than  a  2-inch  gas  pipe,  the  outside  diameter  being 
the  same.  This  rule  does  not  apply  to  hydraulic  tubes,  which 
are  of  considerable  thickness. 

Size  of  Flanges. — Until  quite  recently,  every  engineer  has 
used  his  own  standard  dimensions  for  pipe  flanges.  This  has 
caused  a  great  deal  of  inconvenience,  especially  to  valve  makers. 


80 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


Recently,  however,  the  Engineeering  Standards  Committee,  a 
body  composed  of  members  of  the  Civil  and  Mechanical 
Engineers'  Institutions,  of  the  Institution  of  Naval  Architects, 
and  of  the  Iron  and  Steel  Institute,  drew  up  a  table  settling 
the  diameter  and  thickness  of  flanges,  the  number  of  bolts  to  be 
used,  and  the  pitch  circle;  both  for  steam  pipes  suitable  for 
pressures  up  to  225  Ibs.  per  square  inch,  and  for  exhaust  pipes, 
or  pipes  suitable  for  working  up  to  pressures  not  exceeding  55 
Ibs.  per  square  inch. 

These  tables  are  appended  and  may  prove  useful;  many 
engineering  firms  have  already  adopted  them,  and  others  intend 
to  do  so  as  soon  as  existing  stock  has  been  disposed  of. 


TABLE  XIII. — DIMENSIONS  OF  BRITISH  STANDARD 
PIPE  FLANGES. 

For  steam  pressure  vp  to  225  Ibs.  per  square  inch. 


1  tit  ,  .i-ii  .i  1 

Thickness  of  Flange. 

internal 
diameter  of 
pipe. 

Diameter  of 
Flange. 

Diameter  of 
Bolt  Circle. 

Number 
of  Bolts. 

Diameter 
of  Bolts. 

Cast  iron 
or  Wrought 
Iron. 

Steel  or 
Bronze. 

Inches. 

Inches 

Inches. 

1 

4| 

3/8 

4 

i 

1 

i 

Jl 

5J 

4 

§ 

i 

ii 

54 

4£ 

4 

§ 

f 

Jv 

6^ 

5 

4 

i 

^ 

TB^ 

2| 

7| 

M 

8 

i 

| 

H 

3 

8 

64 

8 

1 

1 

3i 

8i 

7 

8 

| 

1 

f 

4 

9^ 

7i 

8 

I 

ii 

^ 

5 

11 

9| 

8 

1 

li 

i 

6 

12 

10i 

12 

i 

7 

13i 

HJ 

12 

I 

ii 

H 

8 

144 

12f 

12 

1 

l| 

H 

9 

16 

14 

12 

| 

H 

ii 

10 

17 

15 

12              I 

li 

ii 

12 

21* 

17i 

16              | 

if 

i§ 

14 

19| 

16 

i 

if 

li 

16 

24 

20 

i 

i| 

If 

18 

26^ 

24? 

20 

if 

2 

H 

20 

29 

26| 

24 

ii 

2i 

Ii 

24 

331 

30| 

24             11 

2| 

2| 

STEAM    PIPES    AND    VALVES. 


81 


TABLE  XIV. — DIMENSIONS  OF  BRITISH  STANDARD 
PIPE  FLANGES. 

For  steam  pressures  up  to  55  Ibs.  per  square  inch. 


Internal 
diameter  of 
pipe. 

Diameter  of 
Flange. 

Diameter  of 
Bolt  Circle. 

Number 
of  Bolts. 

Diameter 
of  Bolts. 

Thickness  of  Flanges. 

Cast  Iron. 

Steel  or 
Bronze. 

Inches. 

1 

Inches. 
*J 

Inches. 

31 

4 

I 

i 

f 

11 

4| 

3 

4 

4 

1 

H 

61 

3 

4 

| 

2 

6 

*i 

4 

| 

a 

T9* 

24 

*4 

5 

4 

1 

1 

T9* 

3 

71 

5| 

4 

1 

| 

{* 

Si 

8 

6i 

4 

1 

& 

4 

Si 

7 

4 

i 

^ 

H 

5 

10~ 

8i 

8 

i 

i 

H 

6 

11 

91 

8 

1 

i 

H 

7 

12 

101 

8 

I 

i 

I 

8 

131 

111 

8 

i 

1 

9 

l*i 

12| 

8 

| 

i 

10 

16 

14 

8 

§ 

i 

^ 

12 

18 

16 

12 

3 

H 

i 

14 

20| 

18| 

12 

i 

11 

i 

16 

22| 

20| 

12 

1 

11 

i 

18 

25* 

23 

12 

if 

H 

20 

27| 

25i 

16 

I 

14 

U 

24 

324 

29| 

16 

i 

if 

If 

Water  Hammer. — When  steam  at  a  high  pressure  is 
admitted  to  a  pipe  in  which  a  certain  amount  of  cold  water 
is  lying,  a  succession  of  sharp  reports  is  heard,  just  as  though 
blows  were  being  struck  on  the  inside  of  the  pipe  by  a  hammer ; 
sometimes  the  blows  are  sufficiently  strong  to  fracture  the  pipe 
or  a  neighbouring  valve. 

The  precise  action  of  water  hammer  has  recently  been 
investigated  by  Mr.  Strohmeyer,  who  used  glass  tubes  for  his 
investigations.  Mr.  Strohmeyer  found  that  when  steam  was 
admitted  to  a  pipe  containing  water,  waves  were  set  up  in  the 
latter  which  imprisoned  and  isolated  portions  of  the  steam. 
The  isolated  portion  of  the  steam  being  in  contact  with  cold 
water,  and  no  further  supply  of  heat  being  able  to  come  to  its 
rescue,  condensed,  the  water  then  rushed  into  the  space  formerly 
occupied  by  the  steam  and  caused  a  sharp  blow. 

In  addition  to  this  action  of  water  hammer  proper,  the 
presence  of  water  in  a  steam  pipe,  as  explained  earlier  in  this 
chapter  and  illustrated  by  Fig.  27,  may  lead  to  disaster.  In 

6 


82 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


such  a  case  the  water  is  picked  up  by  the  rush  of  steam  and  acts 
as  a  projectile. 

Sometimes  an  accident  is  caused  through  the  attendant  not 
realising  what  goes  on  in  a  steam  pipe  after  the  engine  and 
boiler  valves  have  been  closed.  What  happens  is  this  : — After 
the  valves  have  been  closed  for  some  time,  and  heat  has  been 
lost  by  radiation,  the  steam  condenses  and  forms  water ;  if  the 
valves  are  tight  no  air  can  get  into  the  pipes  and  a  vacuum  is 
formed.  Now,  if  the  drain  cocks  on  the  pipe  are  opened  before 
a  small  quantity  of  steam  is  admitted,  air  will  pass  into  the 
pipes,  but  water  will  not  come  out  against  atmospheric  pressure. 
The  attendant,  seeing  no  water  coming  from  the  pipe,  may 
conclude  that  it  is  free  from  water,  and  may  turn  on  the  steam 
too  suddenly,  with  disastrous  results.  At  an  enquiry  recently 
held  by  the  Board  of  Trade  upon  an  accident  due  to  water 
hammer,  the  attendant  said  that  when  he  opened  the  cocks  he 
saw  no  water  coming  from  the  pipes,  but  heard  a  hissing  noise ; 
even  this  did  not  convey  to  him  the  fact  that  there  was  a 
vacuum  in  the  pipes,  and  that  the  hissing  sound  he  heard  was 
due  to  air  rushing  in. 

Exhaust  Pipes  are  usually  made  of  cast  iron,  as  the  pressure 
they  have  to  withstand  is  low ;  when  the  engine  is  exhausting 
to  the  atmosphere  the  pressure  in  the  pipes  is  not  much  above 
that  of  the  atmosphere.  When  the  engine  is  condensing  the 
pipe  is  subject  to  a  crushing  stress  not  exceeding  15  Ibs.  per 
square  inch. 

In  calculating  the  strength  of  a  wrought-iron  pipe,  we  said 
that  the  weld  and  general  stiffness  of  the  pipe  had  to  be  taken 
into  consideration.  In  a  cast-iron  pipe  there  is  of  course  no- 
weld,  but  in  casting  the  pipe  the  core  may  have  shifted,  and  one 
side  of  the  pipe  may  be  much  thinner  than  the  other.  Even  if 
the  pipe  were  of  the  same  thickness  throughout,  a  pipe  calculated 
on  the  basis  of  the  safe  stress  for  cast  iron  would  be  much  too 
thin,  especially  in  cases  where  the  pressure  is  low.  In  practice 
the  thickness  of  exhaust  pipes  is  approximately  as  follows : — 


Diameter. 

Thickness. 

Diameter. 

Thickness. 

Inches. 

Inch. 

Inches. 

Inch. 

3 

I 

9 

i 

4 

1 

10 

9 
TTT 

5 

| 

12 

| 

6 

7 

T-B 

18 

1 

8 

1 

20 

i 

STEAM    PIPES   AND    VALVES. 


83 


Steam  Traps  are  devices  for  allowing  water  automatically  to 
leave  a  pipe  containing  steam  under  pressure  without  permitting 
steam  to  pass.  Steam  traps  are  of  two  types — viz.,  the  bucket 
and  expansion  types.  The  former  depends  for  its  action  upon 


INLET 


Fig.  31. — Bucket  trap. 

a  floating  bucket;  the  latter  depends  upon  the  difference  of 
temperature  between  the  water  which  has  condensed  and  that 
of  the  live  steam.  Fig.  31  shows  a  bucket  steam  trap  as  made 
by  Messrs.  Holden  &  Brooke,  of  Manchester.  At  starting,  the 
trap  is  partly  filled  with  water,  which  causes  the  floating  bucket 


Fig.  32. — Expansion  trap. 

to  rise  and  close  the  outlet  valve.  When  water  comes  down 
from  the  steam  pipe  it  gradually  fills  the  shell,  reaches  the  top 
of  the  bucket,  and  flows  into  it ;  when  the  bucket  is  full  its 
buoyancy  has,  of  course,  disappeared,  and  the  weight  of  the 


84  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

bucket  is  sufficient  to  open  the  outlet  valve.  The  pressure  of 
steam  on  the  surface  of  the  water  then  drives  the  latter  out 
of  the  bucket,  up  the  central  outlet  pipe ;  when  it  has  been 
expelled  the  weight  of  water  surrounding  the  bucket  causes  the 
latter  to  rise  and  to  again  close  the  outlet  valve. 

Fig.  32  shows  a  Brooke  expansion  trap.  In  this  trap  steam 
can  blow  freely  through  the  central  tube  until  the  steam  has 
warmed  it,  when  the  expansion  of  the  tube  causes  the  outlet 
valve  to  close.  When  water  comes  down  from  the  steam  pipe  it 
cools  the  tube  and  causes  it  to  contract,  consequently  the  valve 
opens  and  the  water  is  blown  out.  A  difference  of  5°  F.  in 
temperature  is  sufficient  to  make  the  trap  act ;  in  fact,  it  is  so 
sensitive  that  if  a  little  water  is  sprinkled  off  the  hand  on  to  the 
central  tube  when  full  of  steam,  the  valve  opens,  spits  out  steam, 
and  then  closes. 

The  objection  to  the  expansion  form  of  trap  is  that  the  valve 
arid  seat  after  a  time  get  cut  by  the  outgoing  steam  and  water, 
and  the  valve  leaks.  In  the  Brooke  bucket  trap  the  outlet 
valve  is  always  under  water,  and  is  given  a  rotary  motion,  so 
that  it  grinds  itself  in  at  every  discharge.  The  bucket  form  of 
trap  is  considered  the  more  reliable  of  the  two  kinds,  but  it 
takes  up  more  space,  and  is  more  expensive  than  the  expansion 
form  of  trap. 

Steam  Stop  Valves. — Fig.  33  shows  a  screw-down,  or  globe 
right-angled  boiler  stop  valve,  having  a  renewable  seating ;  that 
is  to  say,  the  valve  seat  is  not  made  solid  with  the  valve  body, 
but  a  separate  metal  seat  is  forced  in  to  the  body,  and  is  held 
in  by  set  screws.  The  reason  for  making  the  seat  renewable 
is  that  a  valve  seat,  especially  if  the  valve  is  used  for  regulating 
the  flow  of  steam,  gets  scored  or  cut  by  the  action  of  the  steam, 
and  the  valve  is  no  longer  tight  when  closed. 

In  the  stop  valve  illustrated,  the  valve  and  seat  can  be  renewed 
when  scored  without  necessitating  the  renewal  of  the  body. 
The  valves  and  seats  are  usually  made  of  some  alloy  which  does 
not  corrode ;  gunmetal  was  formerly  used,  but  this  alloy  is  not 
suitable  for  superheated  steam.  One  firm  of  valve  makers 
(Messrs.  Templer  &  Eanoe)  use  a  special  nickel  alloy  for  their 
valves  and  seats,  while  another  firm  (Messrs.  Hopkinson)  use  an 
alloy  which  they  call  platnam ;  this  must  not  be  confused  with 
platinum.  Platnam  is  doubtless  a  fancy  name.  The  bodies  of 
valves  for  high-pressure  steam  should  be  made  of  cast  steel  and 
not  of  cast  iron. 

Valves  of  the  screw-down  pattern  should  be  arranged  so  that 
the  steam  assists  the  valve  to  open,  and  constructed  so  that  the 
screw  forces  the  valve  on  to  its  seat ;  if  a  valve  is  constructed  so 


STEAM    PIPES   AND    VALVES. 


85 


that  the  spindle  draws  the  valve  on  to  its  seat,  it  is  difficult 
keep  the  valve  tight. 


Fig.  33. — Screw-down  stop  valve. 

Fig.   34  shows  a  very  good  form  of  valve  of   the  gate,  or 
straight-through  type.     The  valve  shown  is  known  as  the  Stirling 


86 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


(made  by  Templer  &  Ranoe).     It  will  be  seen  from  the  illustra- 
tion that  when  the  valve  is  closed  the  wedges  force  the  two 


ffrf 


to 


w 


rrnrPn    rm    rrnrm 


Fig.  34. — Straight-through  gate  stop  valve. 

valve  faces  outwards  against  the  seatings.  The  two  wedges 
ensure  the  upper  and  lower  portions  of  the  valve  being  forced 
equally  against  the  seats.  The  Stirling  is  a  double-faced  valve — 


STEAM    PIPES    AND    VALVES.  87 

i.e.,  both  valve  faces  make  a  steam-tight  joint.  In  valves  of 
somewhat  similar  type,  suitable  for  water  and  for  exhaust  steam, 
and  known  as  sluice  valves,  one  face  only  of  the  valve  frequently 
makes  the  joint,  but  double-faced  sluice  valves  are  also  made. 

It  is  usual  to  provide  large  valves  of  the  gate  type  with  a 
small  by-pass  valve,  so  that  the  pressure  on  both  sides  of  the 
large  valve  may  be  equalised  before  opening  it.  This  by-pass 
valve  is  useful  for  admitting  a  small  quantity  of  steam  to  the 
pipes  to  warm  them  up,  before  opening  the  main  valves. 

Yalves  of  the  screw-down  globe  pattern  are  generally  used 
upon  boilers  or  engines  where  it  may  be  necessary  to  use  the 
valve  for  regulating  the  flow  of  steam.  Valves  of  the  gate  type 
are  generally  used  in  the  range  of  pipe  to  shut  one  portion  off. 
A  valve  of  the  gate  or  straight-through  type  causes  less  obstruc- 
tion to  the  passage  of  the  steam  than  one  of  the  globe  type. 

Various  forms  of  valve  have  been  designed  to  get  over  the 
cutting  action  of  the  steam ;  they  usually  take  the  form  of  two 
valves  in  one  body,  one  valve  opening  before  steam  can  pass  the 
second ;  the  first  valve  therefore  does  not  get  cut  by  the  action 
of  the  steam.  It  should  be  explained  that  the  cutting  action 
does  not  take  place  when  the  valve  is  fully  open,  but  chiefly 
when  opening,  or  when  the  valve  is  kept  partially  closed  for 
regulating  purposes.  The  "T.  R."  valve  (made  by  Messrs. 
Templer  &  Ranoe)  was  probably  the  first  valve  of  this  kind ;  it 
is  rather  expensive,  but  remains  tight  for  a  very  long  time 
Messrs.  Hopkinson  also  make  a  valve  of  a  somewhat  similar 
kind  which  is  called  the  "centre  pressure"  valve. 

Hopkinson-Ferranti  Valve. — A  steam  valve,  which  has 
recently  been  introduced  and  largely  advertised,  is  the  Hopkin- 
son-Ferranti valve.  The  bore  of  this  valve  is  gradually  reduced 
by  a  nozzle-shaped  body,  until  the  bore  is  only  half  the  diameter 
of  the  pipe  to  which  the  valve  is  attached.  The  valve  itself  is, 
of  course,  at  the  smallest  portion  of  the  bore ;  beyond  the  valve 
proper  the  body  gradually  opens  out  in  a  suitably  proportioned 
manner  until  the  full  area  of  the  pipe  is  again  reached.  The 
principle  upon  which  the  valve  is  constructed,  as  given  by  the 
makers,  is  as  follows: — "Converting  the  pressure  of  the  fluid 
into  velocity  and  reconverting  the  velocity  into  pressure,  thereby 
passing  an  amount  of  steam  equal  to  the  full  carrying  capacity 
of  the  pipe."  The  makers  claim  that  the  valve  is  lighter  and 
cheaper  than  one  having  the  full  opening  of  the  pipe,  that  there 
is  less  risk  of  leakage,  and  that  the  valve  causes  practically  no 
drop  in  the  pressure  of  the  steam  passing  through  it.  The 
author  learns,  from  independent  sources,  that  when  a  single 
valve  of  the  kind  is  employed  the  drop  of  pressure  is  almost 


88  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

negligible,  provided  the  valve  is  fully  opened ;  but  if  the 
attendant  should  fail  to  open  the  valve  to  the  very  fullest 
extent,  so  as  to  cause  the  slightest  obstruction  to  the  steam 
where  it  flows  through  the  throat  of  the  valve  at  a  high  velocity, 
then  the  drop  of  pressure  is  serious. 

It  can  hardly  be  seriously  contended  that  there  is  no  loss  in 
the  conversion  of  pressure  into  velocity  and  reconverting  velocity 
into  pressure.  It  is  difficult  to  imagine  steam  flowing  through 
a  long  corrugated  pipe,  however  accurately  the  corrugations  may 
be  proportioned,  as  freely  as  through'  a  plain  pipe.  Joule's  law, 
given  in  the  chapter  on  steam  turbines,  reads — "When  a  gas 
expands  without  doing  external  work,  and  without  taking  in  or 
giving  out  heat,  its  temperature  does  not  change."  We  know 
that  when  steam  is  passed  through  a  constricted  opening  it 
becomes  slightly  superheated  and  expands,  and  it  is  difficult  to 
see  how  the  makers  can  prevent  the  steam  from  giving  up  some 
of  this  superheat  to  the  valve,  and  so  losing  it  by  conduction 
and  radiation. 

Apart  from  theoretical  considerations  there  is  one  point 
which  should  not  be  overlooked  by  those  using  this  form  of 
valve.  Every  length  of  horizontal  pipe  placed  between  two 
such  valves  should  be  properly  drained,  otherwise  a  pocket  is 
formed  in  which  water  can  lie. 

Isolating  Valves. — Another  form  of  valve  used  on  boilers  in 
power-generating  stations  is  the  isolating  valve.  This  valve  will 
allow  steam  to  leave  a  boiler,  but  will  not  allow  any  to  re-enter. 
Thus  if  several  boilers  deliver  steam  into  one  common  steam 
main,  and  one  boiler  should  develop  a  serious  leak,  or  burst  a 
tube,  that  boiler  only  will  be  put  out  of  action,  if  fitted  with  an 
isolating^  valve,  as  this  valve  will  prevent  steam  entering  from 
the  other  boilers.  Isolating  valves  sometimes  give  trouble  by 
hammering  and  breaking.  This  trouble  is  often  experienced 
when  the  boilers  supply  steam  to  slow-speed  reciprocating 
engines,  and  when  the  pipes  are  of  small  size.  The  explanation 
is  doubtless  as  follows : — When  the  steam  port  of  the  big  slow- 
speed  engine  opens  steam  travels  along  the  pipe  at  a  very  high 
velocity,  the  cut  off  then  suddenly  takes  place,  and  the  flow  of 
steam  at  one  end  of  the  pipe  is  checked ;  the  steam,  which  was 
in  motion  in  the  pipes,  banks  itself  up,  so  to  speak,  and  the 
pressure  rises  higher  than  that  in  the  boiler.  Steam  then  flows 
back  to  the  boiler,  and  closes  the  isolating  valve  with  a  sharp 
blow.  Such  a  blow  repeated  sixty  or  seventy  times  a  minute 
naturally  causes  the  valve  to  collapse  in  a  short  time. 


89 


CHAPTER  VI. 
THE    STEAM    ENGINE. 

(PART  I.) 

IT  is  beyond  the  scope  of  this  book  to  describe  in  detail  a  great 
variety  of  steam  engines  ;  all  that  is  attempted  is  to  make  clear 
the  principles  upon  which  such  engines  work,  and  to  show  how 
the  ordinary  calculations  connected  with  them  are  made. 

Fig.  35  shows  a  vertical  double-acting  engine  with  cylinder 
and  valve  chest  in  section.  The  valve  shown  is  of  the  plain  D 
type,  as  it  is  necessary  to  understand  the  action  of  this  form  of 
valve  before  considering  valves  of  the  piston  or  other  types. 
The  action  of  the  engine  is  this  : — Steam  from  the  boiler  enters 
through  an  inlet  at  the  back  of  the  valve  chest  A ;  the  slide- 
valve  S,  in  the  illustration,  is  just  beginning  to  uncover  the 
steam  port  leading  from  the  valve  chest  to  the  upper  side  of  the 
piston  B ;  the  valve  will  continue  to  move  downwards  until  the 
port  is  fully  open,  when  it  will  begin  to  move  in  an  upward 
direction.  The  pressure  of  steam  on  the  piston  B  causes  it  to 
descend  and  turn  the  crank  C  and  crank  shaft  D  towards  the 
spectator  by  means  of  the  piston-rod  E  and  connecting-rod  F. 

On  the  crank  shaft  is  fitted  an  eccentric  H,  which  gives  motion 
to  the  valve  by  means  of  the  eccentric-rod  J,  and  valve-rod  K. 
The  eccentric  is  set  in  such  a  position,  and  the  valve  is  so 
proportioned,  that  when  the  piston  has  reached  about  three- 
quarters  of  its  downward  stroke  the  slide  valve  will  have 
travelled  upwards  sufficiently  far  to  prevent  the  admission  of 
any  more  steam.  This  is  called  the  point  of  cut-off;  after  the 
cut-off  has  taken  place,  the  steam  which  is  above  the  piston 
expands  and  forces  the  piston  down  to  the  end  of  its  stroke.  A 
flywheel,  L,  is  mounted  on  the  crank  shaft,  the  stored  energy  of 
which  carries  the  crank  over  the  dead  centres  (i.e.,  the  position 
in  which  the  piston  is  at  the  extreme  end  of  the  stroke),  by 
which  time  the  slide  valve  will  be  in  such  a  position  that  it  is 
just  opening  the  port  leading  to  the  underside  of  the  piston ;  the 
slide  valve  will  continue  to  open  the  port  and  the  piston  will 
rise ;  at  three  quarters  of  its  upward  stroke  steam  will  be  cut 
off,  and  the  expansion  of  the  steam  beneath  it  will  cause  the 
piston  to  complete  its  stroke. 


90 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


It  will  be  seen  from  the  illustration  that  when  one  port  is 
uncovered  to  admit  steam,  the  slide  valve  places  the  other  port 


Fig.  35. — Vertical  single-cylinder  engine. 

in  communication  with  the  central  port  or  passage  M,  which,  in 
a  single-cylinder  engine,  leads  to  the  atmosphere  or  condenser, 
and  in  a  compound  engine  to  the  second  cylinder. 


THE    STEAM    ENGINE.  91 

The  port  through  which  the  steam  exhausts  is  closed  by  the 
valve  slightly  before  the  piston  reaches  the  end  of  its  stroke,  so 
that  the  imprisoned  steam  acts  as  a  cushion,  and  helps  to  bring 
the  piston  to  a  standstill,  and  to  restart  it  on  its  reverse  stroke. 
In  a  condensing  engine  this  cushioning  effect  is  lost  to  the 
cylinder  placed  in  connection  with  the  condenser  ;  this  accounts 
for  the  fact  that  certain  high-speed  engines  are  much  more  noisy 
when  condensing  than  when  exhausting  to  the  atmosphere. 

It  may  be  noticed  that  the  faces  of  the  slide  valve  are  wider 
than  the  steam  ports;  the  difference  is  called  the  "lap";  the 
amount  by  which  the  valve  faces  project  beyond  the  ports  in  an 
outward  direction  when  the  valve  is  central,  is  called  "  outside 
lap."  The  amount  of  outside  lap  and  the  position  of  the 
eccentric  regulate  the  point  of  cut-off.  If,  when  the  slide  valve 
is  in  a  central  position,  the  valve  faces  overlap  the  ports  on  the 
inside,  it  is  called  "inside  lap." 

It  will  be  noticed  that,  although  the  piston  has  not  commenced 
its  downward  stroke,  the  valve  has  slightly  uncovered  the  steam 
port;  this  is  called  "the  lead  of  the  valve."  Thus,  if  the  port  is 
opened  by  one-eighth  of  an  inch  when  the  piston  is  still  at  the 
highest  part  of  its  stroke,  the  valve  is  said  to  have  one-eighth 
inch  lead. 

By  cutting  off  the  admission  of  steam  fairly  early,  and  allowing 
it  to  work  expansively,  considerable  economy  is  effected,  but 
with  a  single  D  slide  valve,  if  sufficient  lap  is  given  to  make  the 
cut-off  take  place  before  about  two-thirds  of  the  stroke,  the 
exhaust  is  closed  too  early,  and  too  much  compression  results. 

With  the  D-form  of  slide  valve  the  pressure  of  steam  on  the 
back  of  the  valve  is  more  than  sufficient  to  hold  it  up  against 
the  valve  chest  face.  Thus  a  small  valve,  say  5  inches  by  4 
inches,  has  an  area  of  20  square  inches,  and  if  the  steam  pressure 
in  the  valve  chest  is  120  Ibs.,  a  pressure  of  2,400  Ibs.,  or  over  a 
ton,  will  be  exerted  on  the  back  of  the  valve.  This  great  pres- 
sure renders  the  plain  slide  valve  unsuitable  for  cases  where  the 
steam  pressure  is  high,  or  where  the  engine  runs  at  a  high  speed. 

In  an  engine  designed  for  even  moderately  high  speeds  or 
high  steam  pressures  a  valve  of  the  piston,  or  other  type  to  be 
described  later,  would  be  used.  A  valve  of  the  piston  type  is 
shown  by  Fig.  36.  With  this  form  of  valve  a  liner  is  employed, 
in  which  the  ports  can  be  cut  more  accurately  than  is  possible 
in  the  valve-chest  casting;  bars  are  left,  so  that,  if  rings  are 
used  in  the  piston  valve,  they  will  not  enter  the  ports. 

These  bars  obstruct,  to  a  certain  extent,  the  area  of  the  port, 
and  to  get  the  required  area  a  somewhat  large  piston  valve  is 
necessary.  The  piston  valve  is  usually  made  about  half  the 


92  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

diameter  of  the  cylinder,  and  such  a  large  valve  is  inadmissible 
in  the  case  of  the  low-pressure  cylinder  of  a  big  compound  or 
triple-expansion  engine. 

When  a  plain  slide  valve  is  used  for  the  low-pressure  cylinder 

Fig.  36. — Cylinder  with  piston  valve. 


Fig.  37. — Reversing  gear.  Fig.  37«.  — Setting  eccentrics. 


THE    STEAM    ENGINE. 


93 


of  a  large  engine,  it  is  usually  balanced— i.e.,  the  valve  and  valve 
chest  are  constructed  so  that  the  steam  pressure  acts  upon  a 
small  area  only  of  the  back  of  the  valve.  Fig.  38  shows  the  low- 
pressure  cylinder  of  a  large  vertical  engine,  for  marine  or  land 
use,  fitted  with  a  double-ported  balanced  slide  valve.  In  the 
example  shown,  steam  is  prevented  from  pressing  upon  a  large 


Fig.  38. — Low-pressure  cylinder,  with  balanced  slide  valve, 
assistant  cylinder,  and  metallic  packing. 

portion  of  the  valve  by  means  of  the  ring  R.  This  ring  fits  into 
a  circular  ring  at  the  back  of  the  valve,  and  is  kept  steam  tight 
by  means  of  piston  rings.  The  face  of  the  ring  next  the  valve 
chest  cover  and  the  inside  of  the  latter  are  planed,  the  ring  is 
kept  tightly  up  against  the  valve  chest  cover  by  the  steam 
pressure  acting  at  K.  Should  any  steam  leak  past  the  ring  R, 
it  is  taken  away  to  the  condenser  through  the  opening  C. 

There  are  many  other  methods  of  preventing  the  steam  from 


94  MECHANICAL    ENGINEERING   FOR    BEGINNERS. 

pressing  upon  the  back  of  the  valve.  In  many  cases  the  back 
of  the  valve  is  planed,  and  the  ring  R  rubs  on  it.  Instead  of 
using  piston  rings,  the  ring  II  is  frequently  packed  by  means  of 
a  gland,  as  shown  by  the  inset  at  the  top  of  the  illustration.  In 
the  earlier  methods  of  balancing  a  gland  was  not  employed,  the 
ring  merely  fitted  into  a  recess  in  the  valve  chest  cover,  the 
recess  being  filled  with  asbestos  packing.  This  was  unsatisfac- 
tory, for  if  the  fitter  put  too  much  packing  into  the  recess,  the 
ring  was  forced  down  too  hard  on  to  the  valve ;  if  too  little 
packing  was  used,  steam  leaked  by. 

In  modern  marine  practice  the  valve,  in  addition  to  being 
balanced  in  the  manner  described,  is  usually  provided  with  an 
assistant  cylinder  placed  at  the  top  of  the  valve  chest,  as  shown. 
The  object  of  this  assistant  cylinder  is  not  only  to  carry  the 
weight  of  the  valve,  thus  relieving  the  eccentric  straps,  reversing 
link,  pins,  &c.,  from  the  weight,  but  also  to  assist  the  valve  to 
rise,  and  to  force  it  gently  down  at  the  right  moments.  In  the 
"  Joy"  assistant  cylinder  the  admission  of  steam  is  effected  by  a 
reduction  in  the  size  of  the  valve-rod,  so  that  at  one  portion  of 
the  stroke  steam  is  admitted  to  the  underside  of  the  piston ; 
when  the  piston  has  reached  the  top  of  its  stroke  steam  is 
admitted  to  its  upper  side  by  means  of  slots,  and  the  piston  is 
forced  gently  down. 

In  another  form  of  assistant  cylinder — viz.,  the  "  Lovekin"- 
a  separate  valve  is  used  to  distribute  the  steam  to  the  cylinder, 
and  steam  is  taken   from  the   intermediate   receiver  so  as   to 
obtain  it  at  higher  pressure,  thus  enabling  a  smaller  cylinder 
to  be  used. 

The  assistant  cylinder  is  a  step  in  advance  of  the  plain 
cylinder,  which  was  formerly  fitted  to  the  low-pressure  valve 
chests  of  marine  engines.  This  plain  cylinder  was  open  at 
one  end  to  the  valve  chest,  so  that  steam  pressed  constantly 
on  the  underside  of  the  piston  which  carried  the  main  slide 
valve. 

The  spaces  S  S  in  the  slide  valve  itself  are  open  to  the  steam 
in  the  valve  chest,  so  that  the  valve  is  double  ported.  This 
construction  reduces  the  amount  of  valve  travel  for  a  given  size 
of  port  opening.  In  the  illustration  the  valve  is  fully  open, 
admitting  steam  to  the  upper  side  of  the  piston,  and  placing  the 
lower  side  in  communication  with  the  condenser  through  the 
exhaust  passage  E.  A  facing  piece  of  hard  cast  iron,  in  which 
the  ports  are  accurately  cut,  is  placed  between  the  valve  and  the 
main  casting. 

The  illustration  shows  the  piston-  and  valve-rods  packed  by 
means  of  metallic  packing,  which  will  be  described  later.  The 


OF    THE 

UNIVERSITY 

OF 


THE    STEAM    ENGINE.  95 

cylinder  cover  is  fitted  with  a  spring  relief  valve  shown  at  A. 
The  object  of  this  relief  valve  is  to  allow  any  water  to  escape 
should  such  have  accumulated  on  the  piston.  The  piston  is 
provided  with  a  separate  ring  in  which  the  Ramsbottom  piston 
rings  are  placed.  This  admits  of  the  rings  being  renewed  with- 
out the  necessity  of  removing  the  piston  from  its  rod. 

Reversing.  —  When  the  eccentric  is  fixed  to  the  shaft  in  one 
position,  it  will  allow  the  engine  to  rotate  in  one  direction  only. 
If  the  engine  is  required  to  rotate  in  the  opposite  direction  as 
well,  it  is  necessary  to  have  two  eccentrics,  one  keyed  to  the 
shaft  in  a  position  to  make  the  engine  run  in  one  direction,  the 
second  in  a  position  to  make  it  run  in  the  opposite  direction. 
The  rods  leading  from  the  two  eccentrics  are  connected  to 
opposite  ends  of  a  link.  The  valve  spindle  or  rod  is  connected 
to  a  block  placed  in  the  slotted  link,  the  link  itself  can  be 
shifted  so  that  the  valve-rod  may  be  placed  in  connection  with 
either  of  the  two  eccentrics. 

Fig.  37  shows  the  ordinary  Stephen  son  link  motion.  One 
eccentric  is  in  its  uppermost  position,  and  owing  to  the  position 
of  the  link  it  is  this  eccentric  which  is  actuating  the  valve.  It 
will  be  seen  that  the  lower  port  of  the  steam  chest  (Fig.  36)  is 
uncovered  to  the  steam,  while  the  upper  port  is  in  com- 
munication with  the  exhaust.  If  now  the  link  is  pushed  over 
to  the  left  in  the  direction  of  the  arrow,  the  valve  will  be  drawn 
down  and  the  upper  port  will  be  uncovered  to  the  steam,  while 
the  lower  port  will  be  placed  in  communication  with  the 
exhaust.  The  shaft  in  Fig.  37  is  at  right  angles  to  the  position 
it  would  occupy  if  it  were  working  in  connection  with  the 
cylinder  shown  by  Fig.  36  ;  the  two  views  have  been  so  placed 
to  show  clearly  the  action  of  the  two  eccentrics  and  link,  and 
their  relation  to  the  valve. 

In  actual  practice  the  eccentrics  are  not  set  at  the  highest  and 
lowest  positions  as  shown.  They  are  set  at  an  angle  of  90°,  plus 
the  angular  advance,  in  advance  of,  or  behind  the  crank.  The 
actual  setting  of  the  eccentrics  is  shown  by  Fig.  37  a,  where  the 
diameter  of  the  circle  represents  the  travel  of  the  valve. 
A  =  lap,  B  =  lead.  The  angle  K  is  called  the  "angular 
advance."  A  drawing  similar  to  that  shown  by  Fig.  37a  is  sent 
from  the  drawing  office  to  the  shops  with  the  dimension  A  +  B 
given,  and  the  diameter  of  the  circle.  This  enables  the  fitters 
to  set  the  eccentrics  in  the  correct  positions. 

An  engine  such  as  that  shown  by  Fig.  35  would  not  work 
very  economically  as  regards  steam  consumption,  and  would 
only  be  used  in  cases  where  simplicity  and  small  first  cost 
happened  to  be  of  more  importance  than  economy  of  steam  and 


96  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

fuel.  Such  an  engine  might  be  used  for  driving  a  fan  at  a 
colliery  where  fuel  is  cheap,  or  it  might  be  used  in  cases  where 
the  work  is  intermittent,  and  where  the  somewhat  excessive 
consumption  of  fuel  during  the  short  periods  when  the  engine 
is  at  work  is  of  relative  unimportance. 

A  single-cylinder  engine  working  with  a  steam  pressure  of 
75  Ibs.  and  exhausting  to  the  atmosphere,  would  use  between 
30  and  35  Ibs.  of  steam  for  every  indicated  horse-power  per 
hour ;  so  that  if  the  engine  indicated  50  horse-power,  the  boiler 
supplying  the  steam  would  have  to  evaporate  between  1,500  and 
1,750  Ibs.  of  water  per  hour. 

By  indicated  horse-power,  or  I.H.P.,  is  meant  the  power 
actually  exerted  by  the  steam  in  the  cylinder  without  taking 
into  account  the  engine  friction. 

By  brake  horse-power  (B.H.P.),  or  effective  horse- power,  is 
meant  the  actual  horse-power  delivered  by  the  crank  shaft  of  the 
engine.  Thus  an  engine  giving  100  indicated  horse-power  will 
give  only  about  90  to  95  brake  horse-power,  the  remainder  being 
lost  in  friction.  If  the  engine  gives  90  brake  horse-power  for 
every  100  indicated  horse-power,  its  mechanical  efficiency  is  said 
to  be  90  per  cent.  If  it  gives  95  brake  horse-power  for  every 
100  indicated  horse-power  its  mechanical  efficiency  is  95 
per  cent. 

This  mechanical  efficiency  has  nothing  to  do  with  the 
consumption  of  steam  or  thermal  efficiency.  A  single-cylinder 
engine  which  is  wasteful  in  steam  has  often  a  higher  mechanical 
efficiency  than  a  three-cylinder,  triple-expansion,  condensing 
engine  which  is  very  economical  of  steam.  In  comparing  the 
merits  of  two  engines,  it  is  necessary  to  look  at  the  con- 
sumption of  steam  per  brake  horse -power,  or,  if  the 
consumption  is  given  per  indicated  horse-power,  it  is  necessary 
to  ascertain  the  efficiency  of  such  engine,  and  to  convert  the 
figures  into  consumption  of  steam  per  brake  horse-power.  It 
is  important  to  remember  that  because  an  engine  is  claimed 
to  have  a  very  high  mechanical  efficiency,  it  does  not  follow  that 
it  is  an  economical  engine  to  use.  Before  proceeding  with  the 
study  of  compound  and  triple-expansion  engines,  and  going  into 
the  questions  of  economy  of  steam,  &c.,  it  may  be  well  to  state 
how  the  horse-power  of  a  single-cylinder  engine  may  be  worked 
out. 

The  indicated  horse-power  of  a  double-acting  engine  is  found 
by  multiplying  twice  the  stroke  (in  feet)  by  the  number  of 
revolutions  per  minute,  by  the  area  of  the  piston  in  inches,  and 
by  the  mean  pressure  exerted  on  the  piston,  and  dividing  the 
result  by  33,000.  Stated  as  a  formula  it  is  expressed  thus — 


THE    STEAM    ENGINE.  97 

IHP     =    2xSxRxAxI>. 

33,000 
where  S  =  stroke  in  feet. 

R  =  revolutions  per  minute. 
A  =  area  of  piston  in  inches. 
P  =  mean  pressure  in  Ibs.  exerted  on  the  piston. 

Let  us  take  an  actual  case,  say  of  an  engine  as  shown  by  Fig. 
35,  the  cylinder  of  which  is  10  inches  in  diameter,  the  stroke  8 
inches,  the  number  of  revolutions  300  per  minute,  the  initial 
steam  pressure  as  it  enters  the  cylinder  75  Ibs.,  the  cut-off  takes 
place  at  about  *65  of  the  stroke,  and  the  average  mean  pressure 
on  the  piston  during  the  whole  of  the  stroke  is  50  Ibs. 

As  the  stroke  has  to  be  worked  out  in  feet  or  parts  of  a  foot 
before  we  can  make  the  calculation,  we  must  find  out  what  part 
of  a  foot  the  stroke  of  the  engine — viz.,  8  inches — is.  If  we  turn 
to  the  decimal  equivalents  given  at  the  end  of  the  book,  we  see 
that  8  inches  is  -666  of  a  foot.  We  must  also  find  out  what  the 
area  of  a  10-inch  piston  is ;  this  we  see  from  the  table  of  areas  is 
78-5  square  inches. 

The  calculation  now  is 

2  x  -666  x  300  x  78-5  x  50  _  47.5 
33,000 

The  answer  is  47 '5  I.H.P.  If  the  mechanical  efficiency  of  the 
engine  is  90  per  cent.,  then  the  brake  horse-power  will  be 
47-5  x  90  -r  100  =  42-75  B.H.P. 

To  obtain  an  initial  pressure  of  75  Ibs.  in  the  cylinder  the 
boiler  pressure  would  probably  require  to  be  90  Ibs.  per  square 
inch,  as  there  is  usually  a  drop  of  from  5  to  10  Ibs.  between  the 
boiler  and  engine  stop  valve,  the  drop  depending  upon  the  length 
and  diameter  of  the  steam  pipe,  as  explained  in  the  previous 
chapter.  There  should  also  be  a  difference  of  5  Ibs.  on  the  two 
sides  of  the  engine  governor,  if  good  governing  is  desired. 

Having  got  75  Ibs.  initial  pressure  we  cannot,  however,  count 
on  this  pressure  all  through  the  stroke,  as  after  the  point  of 
cut-off,  which  in  this  case  we  have  assumed  to  take  place  at  -65 
of  the  stroke,  the  steam  expands  and  the  pressure  falls.  The 
question  which  will  naturally  be  asked  by  a  beginner  at  this 
point  is — supposing  the  initial  pressure  is  75  Ibs.,  and  the  cut-off 
•65  of  the  stroke,  how  am  I  to  know  what  the  average  pressure 
of  steam  will  have  been  by  the  time  the  piston  has  reached  the 
end  of  the  stroke1?  Well,  if  the  steam  behaved  like  a  perfect 
gas,  and  expanded  adiabatically — i.e.,  without  receiving  heat  from, 
or  giving  up  heat  to,  the  cylinder  walls — the  answer  would  be 

7 


98  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

simple,  for,  according  to  Boyle's  law,  the  volume  of  a  gas  varies 
inversely  as  its  pressure,  the  temperature  being  kept  constant. 
Thus  if  a  given  quantity  of  gas  expands  to  twice  its  volume,  its 
pressure  falls  to  half  the  original  pressure ;  if  expanded  to  four 
times  its  original  volume,  the  pressure  falls  to  one-quarter  of  what 
it  was  originally.  This  law  is  expressed  thus,  P  x  Y  =  constant, 
for  if  we  have  gas  at  100  Ibs.  pressure,  the  volume  of  which  is, 
let  us  say,  4  cubic  feet,  and  we  multiply  the  pressure  by  the 
volume,  the  answer  is  400 ;  if  we  expand  the  same  quantity  of 
gas  into  a  space  of  8  cubic  feet,  its  pressure  will  fall  to  50  Ibs., 
but  multiplying  the  pressure  and  volume  together,  we  still  get 
400  as  the  answer.  Therefore,  pressure  multiplied  by  volume 
equals  a  constant.  This  fall  of  pressure  is  represented  by  a 
hyperbolic  curve,  which  the  reader  will  be  shown  how  to  construct. 

The  case  of  steam  is  not  so  simple  as  that  of  a  perfect  gas,  but 
it  follows  approximately  the  same  law.*  The  true  adiabatic 
curve  for  steam  which  is  initially  dry,  falls  slightly  below  the 
hyperbolic  curve,  but  the  effect  of  initial  condensation  and  re- 
evaporation  (referred  to  later),  causes  the  expansion  line  of 
diagrams  taken  from  actual  engines,  to  approximate  closely 
to  the  hyperbolic  curve.  To  construct  this  curve  proceed  as 
follows: — 

Let  A  B  (Fig.  39)  represent  the  stroke  of  the  engine,  also  the 
line  of  absolute  vacuum,  and  A  C  the  absolute  pressure  of  the 
steam  (the  meaning  of  absolute  pressure  was  explained  on  p.  47). 
Complete  the  rectangle  as  shown,  which  will  then  represent  the 
volume  of  the  cylinder.  Mark  D  at  the  point  of  cut-off,  and 
draw  the  line  D  E.  Divide  E  H  into  equal  divisions,  1,  2,  3, 
&c.,  and  let  fall  perpendiculars  to  the  line  A  B.  Draw  lines 
from  the  point  A  to  the  points  1,  2,  3,  4,  &c.,  and  from  the 
points  where  these  lines  cut  E  D  at  x,  x,  x  draw  horizontals 
cutting  the  perpendiculars  1,  2,  3,  &c.,  at  the  points  z,  z,  z;  then 
the  curve  E  M  drawn  through  these  points  is  a  hyperbola.  The 
atmospheric  line  is  drawn  14*7  Ibs.  above  the  zero  line. 

Fig.  39  shows  how  the  steam  pressure  falls  if  the  cut-off  takes 
place  at  *25  of  the  stroke.  Figs.  40  and  41  show  the  fall  of 
pressure  when  the  cut-off  takes  place  at  P5  and  -75  respectively. 

In  constructing  the  above  diagrams  we  have  neglected  the 
effect  of  clearance — i.e.,  the  steam  passages  and  space  between 
the  piston  and  cylinder  cover  when  the  piston  is  at  the  end  of 
its  stroke.  If  these  clearances  are  to  be  taken  into  account, 

*  The  expression  for  the  relation  between  the  pressure  and  volume  of 
saturated  steam  is  P  x  V*  =  a  constant,  where  the  value  of  x  depends  on 
the  dryness  of  the  steam.  The  expression,  P  V-f£  =  constant,  is  frequently 
used  for  saturated  steam. 


THE    STEAM    ENGINE. 


99 


their  volume  must  be  represented  by  the  rectangle  shown  by 
dotted  lines  in  Fig.  41,  the  point  A  being  set  back  accordingly. 

From  a  hyperbolic  curve  it  is  easy  to  find  the  mean  pressure 
theoretically   exerted    by    the    steam    throughout    its    stroke, 


too 


Zero  Line 


Fig.  40. 


Fig.  41, 


assuming  P  V  =  constant.  Tables  giving  the  mean  pressure, 
obtained  with  various  points  of  cut-off,  are  given  in  many 
engineering  pocket-books  arid  text-books,  but  the  mean  pressures 


100 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


so  given  are,  as  a  rule,  much  higher  than  are  found  in  actual 
practice. 

In  the  diagrams  given  it  is  assumed  that  the  steam  continues 
to  enter  the  cylinder  at  its  full  initial  pressure  right  up  to  the 
point  of  cut-off,  but  in  real  engines,  especially  in  small  fast- 
running  engines,  the  steam  gets  throttled  in  the  passages,  and 
there  is  a  falling-off  in  pressure  before  cut-off  takes  place.  Again 
the  diagrams,  as  shown,  assume  the  steam  to  be  passed  away 
without  causing  any  back  pressure  ;  also  the  effect  of  compression 
is  not  taken  into  account.  In  the  indicator  diagrams  taken 
from  an  actual  engine,  the  mean  pressure  which  is  really  exerted 
by  the  steam  is  seen,  and  the  data  obtained  from  such  diagrams 
usually  serve  as  a  guide  when  designing  a  new  engine.  The 
following  Table  may  be  instructive.  Column  A  gives  the  mean 
pressures  which  should  be  obtained,  according  to  a  well-known 
engineering  pocket-book,  with  various  cut-offs,  and  with  an 
initial  steam  pressure  of  80  Ibs.  ;  column  B  gives  the  mean 
pressures  which  should  be  obtained,  according  to  rules  given  in 
a  good  book  on  the  steam  engine ;  while  column  C  gives  the 
mean  pressures  which  were  actually  obtained  in  a  good  high- 
speed engine,  indicating  about  40  indicated  horse-power  at  full 
load. 

INITIAL  PRESSURE  80  LBS.  ABOVE  ATMOSPHERE. 


Point  of 
Cut-off. 

A. 
Mean 
Pressure. 

B. 

Mean 
Pressure. 

C. 
Mean 
Pressure. 

•25 

477 

32 

30 

•375 

59-5 

44 

39 

•5 

67'7 

53i 

46 

•625 

73-5 

63 

51 

•75 

77-3 

71 

56 

It  is,  of  course,  quite  possible  that  the  mean  pressure  given 
in  column  B  might  be  obtained  in  a  slow-speed  engine  having 
steam  passages  of  ample  size,  but  those  given  in  column  A  could 
only  be  obtained  in  a  theoretically  perfect  engine,  and  must  be 
treated  accordingly. 

We  have  seen  how  the  horse-power  of  an  engine  is  worked 
out,  if  the  mean  pressure  and  all  the  other  factors  are  known. 
If  we  wish  to  know  what  the  mean  pressure  must  be  to  give  a 
certain  power  when  the  speed  and  other  particulars  are  known, 

'.         ,    .      -p      33,000  x  H.P. 
the  formula  is — P  = 


THE    STEAM    ENGINE.  101 

If  the  speed,  mean  pressure,  and  length  of  stroke  are  known, 
and  we  wish  to  find  out  what  area  the  piston  must  have  in  order 

.    .  33,000  x  H.R 

to  give  a  certain  horse-power,  the  tormula  is  —  A  =  -    QT?P  -  • 

The  formula  is  the  same  in  each  case,  but  transposed. 

For  working  out  the  power  of  a  single-acting  engine  —  i.e.,  one 
in  which  the  steam  acts  on  one  side  of  the  piston  only  —  the 

.    .       TTT^       SRAP 
formula  1S-H.P.  = 


As  already  explained,  one  horse-power  is  the  power  required 
to  raise  33,000  Ibs.  1  foot  high  in  one  minute,  or  1  Ib.  33,000  feet 
high  in  one  minute. 

We  have  said  that  the  single-cylinder  engine  is  uneconomical. 
Why  is  this,  and  can  it  be  made  to  work  economically'? 

In  the  first  place,  with  the  D-form  of  slide  valve,  or  with  a 
single  piston  valve,  the  point  of  cut-off  cannot  well  be  made 
earlier  than  J  of  the  stroke,  so  that  when  the  piston  has  com- 
pleted its  stroke  a  large  quantity  of  steam  at  a  fairly  high 
pressure,  and  capable  of  doing  further  work,  is  passed  away  to 
the  atmosphere,  and  is  lost,  or,  in  other  words,  so  much  heat  is 
wasted  (see  Fig.  41).  But  suppose,  instead  of  the  D-valve,  we 
take  the  case  of  an  engine  fitted  with  the  Corliss  or  Drop  type 
of  valve,  which  will  allow  the  cut-off  to  take  place  at  any  part 
of  the  stroke  without  interfering  with  the  exhaust,  and  make 
the  cut-off  take  place  at  J  or  J  of  the  stroke,  thus  allowing  the 
steam  to  work  expansively  (see  Fig.  39).  Would  this  not  be 
economical  1 

It  would  not,  for  these  reasons.  If  we  cut  off  the  admission 
at,  say,  ^  of  the  stroke,  and  expand  the  steam  until  the  end  of 
the  stroke,  the  steam,  when  it  leaves  the  cylinder,  is  much 
cooler  than  it  was  when  it  entered  ;.  it  then  cools  down  the 
ports,  cylinder  walls,  and  piston,  and  a  certain  proportion  of 
the  incoming  steam  at  the  next  stroke  on  coming  in  contact 
with  the  ports,  cool  walls,  and  piston,  condenses  and  turns  into 
water.  This  is  called  initial  condensation,  and  is  responsible 
for  a  considerable  loss  of  heat  or  energy.  * 

*  The  loss  of  heat  from  the  cylinder  walls  is  due  not  only  to  the  fact  of 
their  having  been  in  contact  with  the  cooler  steam,  but  also  to  the  circum- 
stance that  when  the  steam  has  condensed  on  the  cylinder  walls  it  remains 
there  as  a  film  of  water.  Now,  after  cut-off  takes  place,  the  steam  expands 
and  the  pressure  falls;  the  film  of  water  is  then  in  a  position  to  evaporate. 
It  will  have  been  gathered  from  the  chapter  on  "  Steam  Raising"  that  the 
temperature  at  which  steam  is  formed  depends  on  the  pressure.  Now,  the 
pressure  having  fallen,  the  water  robs  the  walls  of  the  heat  which  had 
previously  been  given  to  them,  and  re-evaporates.  This  loss  has  to  be 
made  good  by  the  incoming  steam  at  the  next  stroke. 


102  MECHANICAL    ENGINEERING   FOR    BEGINNERS. 

From  data  in  his  possession,  the  author  finds  that  a  good 
single-cylinder  non-condensing  engine  of  the  high-speed  type, 
working  with  75  Ibs.  initial  gauge  pressure  and  '7  cut-off,  used 
31|  Ibs.  of  steam  per  I.H.P.  per  hour;  while  with  the  same 
pressure  and  an  extremely  early  cut-off — viz.,  -2 — the  engine 
used  32  Ibs.,  so  that  the  early  cut-off  was  actually  harmful. 
The  best  point  of  cut-off  for  this  engine  was  '5,  when  it  used 
29J  Ibs.  of  steam  per  I.H.P.  per  hour.  It  should  be  remem- 
bered, too,  that  a  good  high-speed  engine  suffers  less  from  initial 
condensation  than  a  slow-speed  engine,  as  in  the  latter  there  is 
more  time  for  the  interchange  of  heat  to  take  place  between 
the  periods  of  admission. 

This  initial  condensation  prevents  one  from  obtaining  much 
advantage  from  taking  the  exhaust  steam  from  a  single-cylinder 
engine  to  a  condenser,  as  the  ports  and  cylinder  walls  are 
subjected  to  a  still  lower  temperature  than  when  non- 
condensing. 

Cushioning  in  a  single-cylinder  non-condensing  engine,  and 
in  the  H.P.  cylinder  of  a  compound  condensing  engine  has  a 
beneficial  effect  on  the  economy  of  the  engine,  as  not  only  is 
a  certain  volume  of  steam  saved,  but  also  the  temperature  of  the 
compressed  steam  is  raised,  and  the  initial  condensation  of  the 
incoming  steam  is  reduced. 

To  use  steam  to  its  greatest  advantage  we  need  to  have  it  at 
as  high  a  temperature  as  possible  (within  limits),  to  make  it  do 
as  much  useful  work  as  possible,  during  which  time  it  will  be 
giving  up  its  heat,  and  then  to  get  rid  of  it  without  cooling 
down  the  incoming  steam  more  than  is  necessary.  The  greatest 
ideal  efficiency  of  a  steam  engine  is 

Tl  -  T2. 
Tl 

where  Tl  =  temperature  of  steam  supplied. 
T2  =  „  „         rejected. 

The  conditions  just  mentioned  can  be  more  nearly  complied 
with  in  a  steam  turbine  than  in  a  reciprocating  engine.  To 
approach  them  in  the  latter  we  need  two,  three,  or  even  four 
cylinders. 

A  compound  engine  is  one  in  which  the  expansion  of  the 
steam  is  carried  out  in  a  pair,  or  pairs  of  cylinders.  In  a  two- 
cylinder  compound  engine  there  is  one  high-pressure  (H.P.) 
cylinder  and  one  low-pressure  (L.P.)  cylinder.  The  steam,  after 
doing  its  work  in  the  H.P.  cylinder,  instead  of  being  passed 
away  to  the  atmosphere  or  condenser,  is  taken  to  a  recep- 
tacle or  receiver,  and  from  this  receiver  the  L.P.  cylinder 


THE    STEAM    ENGINE.  103 

draws  its  steam.  The  L.P.  cylinder  is  much  larger  in  diameter 
than  the  H.P.  cylinder,  the  area  of  the  L.P.  piston  being  usually 
three  or  three  and  a  half  times  greater  than  that  of  the  H.P. 
piston.  As  soon  as  the  valve  of  the  L.P.  cylinder  opens,  steam 
enters  from  the  receiver  and  continues  to  do  so  until  the  cut-off 
takes  place,  when  the  steam  expands  and  forces  the  piston  to 
the  end  of  its  stroke ;  the  exhaust  port  then  opens,  and  the 
steam  is  either  passed  to  the  atmosphere  or  to  the  condenser. 
Thus  the  H.P.  cylinder  is  never  cooled  down  by  the  com- 
paratively low  final  temperature  of  the  out-going  steam. 

When  the  steam  is  exhausted  to  the  open  air  it  has  to  be 
discharged  against  the  pressure  of  the  atmosphere,  which  is 
about  14-7  Ibs.  (temperature  of  the  steam,  212°  R).  If,  however, 
the  steam  is  condensed  by  means  of  cold  water,  a  vacuum  is 
formed,  and  the  steam  is  discharged  against  an  absolute  pressure 
of  1  or  2  Ibs.  only  (the  exact  pressure  depends  on  the  condensing 
arrangements  and  size  of  the  exhaust  pipe),  and  at  a  temperature 
of  100°  to  125°.  Thus,  more  work  is  got  out  of  the  steam  than 
can  be  obtained  when  discharging  against  atmospheric  pressure. 

In  the  earliest  form  of  steam  engine  steam  was  used  for 
raising  the  piston  only.  When  the  piston  reached  the  top  of  its 
stroke  a  jet  of  water  was  squirted  in,  a  vacuum  was  created,  and 
the  atmospheric  pressure  forced  the  piston  down.  The  initial 
condensation  must,  of  course,  have  been  enormous. 

In  a  triple-expansion  engine  the  expansion  of  the  steam  is 
carried  out  in  three  cylinders ;  in  such  an  engine  the  range  of 
temperature  in  each  cylinder  is  still  less  than  in  a  compound 
engine,  and  the  loss  from  initial  condensation  is  therefore  still 
further  reduced. 

The  consumption  of  steam,  not  superheated,  in  a  good  compound 
engine  of  the  Corliss  type,  when  condensing,  is  between  14  and 
15  Ibs.  of  steam  per  I.H.P.  per  hour.  When  exhausting  to  the 
atmosphere  the  consumption  is  between  19  and  22  Ibs.  per  I.H.P. 
per  hour. 

The  consumption  of  steam  in  a  good  triple-expansion  engine 
when  condensing  is  between  12J  and  13.^  Ibs.  per  I.H.P.  per 
hour.  When  exhausting  to  the  atmosphere  the  consumption  is 
between  18 J  and  19 J  Ibs.  per  I.H.P.  per  hour,  assuming  the 
steam  pressure  is  not  less  than  180  Ibs.  above  the  atmosphere. 
With  steam  pressures  lower  than  this  there  is  but  little  advan- 
tage in  employing  a  triple-expansion  engine,  when  it  has  to 
work  non-condensing. 

The  consumption  of  steam  in  a  quadruple-expansion  engine  is 
slightly  less  per  I.H.P.  than  in  a  triple-expansion  engine,  but  it 
is  somewhat  doubtful  whether  the  gain  through  carrying  out  the 


104  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

expansion  in  four  stages  instead  of  three  is  sufficiently  great  to 
compensate  for  the  extra  first  cost,  and  for  the  extra  friction  due 
to  the  use  of  the  fourth  cylinder. 

In  marine  engines,  which  are  usually  tested  for  economy  after 
erection  on  board  ship,  the  economy  or  otherwise  is  usually 
referred  to  in  Ibs.  of  coal,  as  it  is  rather  a  troublesome  matter  to 
weigh  or  measure  the  water  used.  The  average  consumption  of 
coal  on  modern  battle-ships  having  triple-expansion  engines  is 
1*78  Ibs.  of  coal  per  I.H.P.  per  hour  at  the  most  economical  load 
— viz  ,  about  70  per  cent,  of  the  full  power,  and  1-92  Ibs.  of  coal' 
at  full  power. 

In  the  Britannia,  where  superheated  steam  was  used,  the  con- 
sumption of  coal  was  1-5  and  1-85  Ibs.  respectively. 

These  consumptions  of  coal  include  the  amount  burnt  for 
making  steam  for  the  auxiliary  machinery.  In  the  mercantile 
marine,  where  there  is  less  auxiliary  machinery,  the  consumption 
of  coal  is  from  1*3  to  1*75  Ibs.  per  I.H.P.  per  hour. 

The  consumption  of  coal  in  locomotive  engines,  as  tested  in 
America  by  means  of  a  dynamometer,  varied  from  3*5  to  4-5  Ibs. 
per  B.H.P.  in  simple  engines  for  goods  traffic,  and  from  2  to 
3*7  Ibs.  in  compound  engines  for  the  same  class  of  work.  The 
consumption  of  coal  in  simple  passenger  locomotives  varied  from 
3-5  to  5-0  Ibs.,  and  in  compound  passenger  locomotives  from 
2-2  to  5*0  Ibs.,  the  higher  consumption  of  coal  always  occurring 
at  high  speeds. 

In  a  mill  having  a  good  compound  condensing  engine  a  con- 
sumption of  1*5  to  1*75  Ibs.  of  coal  per  I.H.P.  is  regarded  as  a 
very  fair  performance. 

Corliss  Gear. — We  have  already  spoken  of  an  engine  fitted 
with  the  Corliss  type  of  valve  gear,  by  means  of  which  the  cut- 
off can  be  made  early  without  interfering  with  the  opening  of 
the  exhaust  port  at  the  proper  time.  Fig.  42  shows  the  Spencer- 
Inglis  form  of  this  gear  in  elevation,  and  Fig.  43  shows  a  section 
through  the  L.P.  cylinder  of  a  large  Corliss  engine  recently 
supplied  by  Messrs.  Fullerton,  Hodgart  &  Barclay,  of  Paisley, 
for  the  East  Bandt  Gold  Mining  Company. 

The  Corliss  form  of  valve  gear  was  invented  by  an  American 
engineer,  whose  name  it  bears.  It  has  been  very  largely  used 
in  England  and  America,  but  is  only  suitable  for  engines  running 
at  a  speed  of  1 20  revolutions  per  minute  or  less. 

It  will  be  seen  from  Figs.  42  and  43  that  there  are  separate 
admission  and  exhaust  valves  at  each  end  of  the  cylinder ;  by 
this  arrangement  long  steam  ports  are  avoided,  and  the  relatively 
cool  exhaust  steam  does  not  pass  through  the  admission  ports,  as 
is  the  case  with  an  engine  having  a  single  D-slide  valve.  The 


THE    STEAM    ENGINE. 


105 


exhaust  ports,  which  are  at  the  lowest  portion  of  the  cylinder, 
allow  any  water  which  has  not  re-evaporated  to  be  swept  out  by 
the  piston  at  every  stroke. 


B      B 


Fig.  43. — L.P.  cylinder,  with  double-ported  Corliss  valves. 

The  Corliss  gear  may  perhaps  look  complicated  to  the  beginner, 
but  really  it  is  very  simple.     The  steam-admission  valves  at  the 


106  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

top  of  the  cylinder  are  kept  closed  by  the  rods  A  A,  Fig.  42. 
Each  of  the  rods  is  connected  to  a  plunger  inside  the  small 
cylinders  and  dashpots  placed  back  to  back  at  B  B.  The  plungers 
usually  are  drawn  in  by  means  of  springs,  and,  less  frequently, 
by  placing  one  side  of  the  plunger  in  communication  with  the 
condenser,  and  leaving  the  other  side  open  to  the  atmosphere. 
The  effect  is  to  keep  the  main  steam-admission  valves  closed 
until  one  of  the  blocks,  C,  which  is  connected  to  a  lever  fixed  to 
the  valve  spindle,  is  taken  hold  of  by  the  catch  plates  D  D  ;  the 
motion  of  these  catch  plates  and  the  block  they  have  caught 
hold  of  causes  the  steam  valve  to  open.  When  the  piston  has 
reached  a  certain  position  the  catch  plates  are  opened  outwards, 
the  block  C  is  released,  and  the  main  steam  valve  immediately 
closes. 

The  mechanism  which  causes  the  plates  to  open  outwards  is 
shown  by  the  inset.  The  releasing  toe  is  pivoted  to  a  prolonga- 
tion of  the  block  C  already  referred  to,  and  when  drawn  along 
with  it — the  end  of  the  lever  attached  to  the  toe  being  prevented 
from  moving  inwards  by  the  rod  R — the  toe  is  forced  to  take 
up  a  more  oblique  position  across  the  catch  plates,  and  opens 
them  outwards.  The  rod  R,  is  also  connected  to  the  governor, 
so  that  if  the  engine  runs  too  fast  the  toe  is  forced  into  its 
oblique  position  a  little  earlier  in  the  stroke ;  the  point  of  cut-off 
is  thus  regulated  by  the  governor. 

The  catch  plates  D  are  worked  from  the  wrist-plate  F,  which 
receives  its  oscillating  motion  from  an  eccentric,  or  eccentrics,  in 
the  ordinary  way.  The  exhaust  valves  have  no  trip  gear  like 
the  steam  valves ;  they  are  connected  by  the  rods  H  H  to  a 
second  wrist-plate  placed  behind  the  first,  and  are  unaffected 
by  the  action  of  the  governor.  Sometimes  both  steam  and 
exhaust  valves  are  worked  off  the  same  wrist-plate,  but  a 
greater  range  and  more  accurate  setting  can  be  obtained  with 
two  wrist-plates. 

The  sectional  view  (Fig.  43)  is  almost  self-explanator.  The 
cylinder  shown  is  40  inches  in  diameter,  has  a  5  feet  stroke,  and 
is  steam-jacketed.  The  speed  is  51  revolutions  per  minute. 
The  valves  are  double  ported.  Steam  from  the  H.P.  cylinder 
enters  at  the  far  side,  and  passes  up  the  passage,  which  forms 
part  of  the  receiver,  shown  by  dotted  lines.  The  entrance  to 
this  passage  is  beyond  the  exhaust  opening,  and  is,  of  course, 
separated  from  it.  The  steam  valve  at  the  top  right-hand  end 
has  been  drawn  open  by  the  catch  plates,  while  the  steam  valve 
at  the  top  left-hand  end,  which  had  previously  been  tripped,  is 
closed.  The  exhaust  valve  at  the  bottom  right-hand  end  is 
closed,  while  the  exhaust  valve  at  the  other  end  is  open.  The 


THE    STEAM    ENGINE. 


107 


opening  K  is  for  the  purpose  of  draining  the  jacket.  The 
admission  to  the  jacket  is  at  the  far  side  of  the  cylinder,  and  is 
not  shown.  The  glands  of  the  piston  and  tail  rods  are  packed 
with  a  patent  metallic  packing,  which  is  described  later. 


44. — Drop  valve  engine. 


Fig.  44a. — Van  der  Kerchove  engine. 


108  MECHANICAL    ENGINEERING   FOR    BEGINNERS. 

Drop,  or  Double-beat  Valves. — Another  form  of  valve, 
which  is  considered  by  some  engineers  to  be  more  suitable  for 
use  with  superheated  steam  than  the  Corliss  valve,  is  the  double- 
beat  valve.  The  L.P.  cylinder  of  an  engine  fitted  with  these 
valves  is  shown  by  Fig.  44.  The  valves,  which  are  of  the 
equilibrium  type,  are  usually  worked  by  some  form  of  trip  gear 
actuated  by  eccentrics  placed  on  a  shaft  running  alongside  the 
cylinder,  and  parallel  to  the  piston-rod.  A  considerable  number 
of  engines  fitted  with  this  form  of  valve  has  been  made  in  this 
country  and  abroad.  The  chief  objection  to  them  is  that  the 
valves  close  with  a  somewhat  heavy  blow,  causing  wear  and 
tear,  and  resulting  in  leaky  valve  seatings.  Another  objection 
is  that  the  clearances  are  rather  greater  than  is  the  case  with 
the  Corliss  form  of  valve.  To  overcome  these  objections  Messrs. 
Van  der  Kerchove,  a  well-known  firm  of  Belgian  engineers, 
have  brought  out  and  patented  the  engine  shown  by  Fig.  44a, 
in  which  piston  valves  are  used  instead  of  double-beat  valves. 
The  piston  valves  are  worked  by  trip  gear ;  they  are  provided 
with  piston  rings  and  work,  of  course,  in  liners  in  which  ports 
are  cut.  The  piston  valves  have  lap,  so  that  the  cushioning  of 
the  valve  is  effected  after  it  has  closed  the  port.  The  clearances, 
as  will  be  seen  from  the  illustration,  are  extremely  small.  This 
type  of  engine  is  made  in  England  by  Messrs.  Musgrave,  of 
Bolton. 

The  Lentz  valve  gear  consists  of  valves  of  the  double-beat 
type,  shown  by  Fig.  44.  The  valves  are,  however,  not 
worked  by  any  form  of  trip  gear,  but  take  their  motion  from 
a  rocking  cam  actuated  by  eccentrics,  and  the  usual  link- 
reversing  gear. 

The  Willans  Central-valve  Engine. — An  engine  of  an 
entirely  different  type,  and  one  which  has  had  a  great  vogue  in 
electric  generating  stations  in  this  country,  is  the  high-speed 
Willans  central- valve  engine,  illustrated  diagrammatically  by 
Fig.  45.  The  illustration  shows  a  three-crank  triple-expansion 
engine,  the  action  of  which  is  briefly  this — Steam  enters  the 
steam  chest  A,  and  passes  through  ports  cut  in  the  trunk  or 
hollow  piston-rod  T,  when  the  latter  is  at  the  top  of  its  stroke. 
Moving  up  and  down  inside  each  trunk  is  a  line  of  piston  valves 
worked  by  an  eccentric  placed  on  the  crank  pin.  These  piston 
valves  control  the  admission  of  steam  to  the  cylinder,  and  the 
exhaust  from  the  upper  to  the  lower  side  of  the  piston,  the  space 
below  each  piston  being  the  receiver  R.  The  steam,  after  having 
done  its  work  in  the  H.P.  cylinder  B,  is  passed  to  the  underside 
of  the  piston  or  receiver  R.  From  this  receiver  the  inter- 
mediate cylinder  D  draws  its  steam,  and  in  turn  passes  it  on 


THE    STEAM    ENGINE. 


109 


to  its  receiver  R1?  and  thence  to  the  L.P.  cylinder  E;  after 
leaving  the  L.P.  cylinder,  the  steam  enters  the  exhaust  chamber 
G,  which  is  placed  in  communication  either  with  the  atmos- 
phere or  with  the  condenser. 


Fig.  45. — Triple-expansion  three-crank  Willans  engine. 

A,  Steam  chest.  H,  Air  chamber. 

B,  H.P.  cylinder.  K,  Guide  piston." 
R,  H.P.  receiver.  T,  Trunks. 

D,  Intermediate  cylinder.  L,  Eccentrics  and  straps. 
R,  Intermediate  receiver.  M,  Connecting-rods. 

E,  L.P.  cylinder.  P,  Piston. 
G,  Exhaust  chamber. 

The  Willans  engine  is  single-acting — i.e.,  the  steam  acts  on  one 
side  of  the  piston  only,  so  that  there  is  no  push-and-pull  action 
on  the  brasses.  In  order  to  prevent  a  pull  coming  on  the 
brasses  on  the  up-stroke  due  to  the  momentum,  or  more  scienti- 


110 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


fically  expressed,  to  the  inertia  *  of  the  moving  parts,  a  separate 
chamber  H  is  provided,  in  which  air  is  compressed  during  the 
upward  stroke,  the  greater  portion  of  the  power  expended  in 
compressing  the  air  being  given  out  on  the  downward  stroke. 
The  piston  K,  which  compresses  the  air,  serves  also  as  the  cross- 
head  and  guide.  The  crank  chamber  below  the  crank  shaft  is 
partly  filled  with  oil,  which  is  splashed  up  over  the  journals  and 
into  the  guide  cylinders. 

The  horse-power  of  some  standard  Willans  engines  and  the 
speeds  at  which  they  run  are  as  follows  : — 

TABLE   XV. 


Makers' 
Distinguishing 
Letter. 

I 

H.P. 

Revolutions  per 
Minute. 

3F 

90 

to     100 

470 

3G 

120 

150 

460 

3H 

200 

240 

380 

31 

300 

350 

350 

3Q 

400 

450 

340 

3R 

500 

575 

320 

3S 

600 

700 

300 

3T 

750 

825 

270 

3V 

1,000 

1,250 

230 

3X 

2,000 

2,500 

200 

Although  the  number  of  revolutions  at  which  the  Willans 
engine  runs  is  very  high,  yet  the  piston  speed,  owing  to  the 
short  stroke,  is  comparatively  low.  The  piston  speed  ranges 
from  470  feet  per  minute  in  the  case  of  the  100  I. H.P. 
engine  to  787  feet  in  the  case  of  the  largest  engine  of  all. 
These  speeds  compare  favourably  with  those  of  many  large 
marine  engines  and  locomotives ;  for  instance,  the  piston  speed 
of  the  engines  of  H.M.S.  Africa  is  1,124  feet  per  minute  when 
running  at  full  speed — viz.,  128  revolutions  per  minute;  and  920 
feet  when  running  at  cruising  speed — viz.,  115  revolutions. 
The  piston  speed  in  a  modern  locomotive  having  cylinders 
19 1  inches  diameter  by  26  inches  stroke  and  6-feet  6-inch  driving- 
wheels,  is  1120  feet  when  running  at  60  miles  per  hour.  The 

*  It  should  be  explained  that  a  body  requires  a  certain  force  to  set  it  in 
motion  ;  when  once  set  in  motion  the  body  will  continue  to  move  in  a 
straight  line  until  stopped  by  gravity  or  by  some  other  force.  This 
unwillingness  to  start  and  unwillingness  to  stop  when  once  set  going  is 
called  the  inertia  of  the  body. 


THE    STEAM    ENGINE.  Ill 

piston  speeds  of  large  horizontal  Corliss  engines  are  usually  about 
500  feet  per  minute. 

Belliss  and  Browett-Lindley  Engines. — Two  other  well- 
known  high-speed  engines  are  those  of  Messrs.  Belliss  and 
Browett-Lindley.  These  engines  are  double-acting,  the  principle 
being  the  same  as  that  of  the  engine  shown  by  Fig.  35.  The 
engines,  however,  are  usually  made  either  compound  or  triple- 
expansion.  The  compound  engine  has  one  high-pressure  and 
one  low-pressure  cylinder  placed  side  by  side  and  two  cranks ; 
the  triple  engine  has  three  cylinders  placed  side  by  side  and 
three  cranks.  It  may  be  here  remarked  that  a  three-crank 
engine,  especially  if  the  pistons  are  approximately  of  the  same 
weight,  causes  much  less  vibration  than  any  two-crank  engine ; 
for  this  reason,  three-crank  engines  are  usually  selected  for  those 
electric  generating  stations  which  are  surrounded  by  dwelling- 
houses.  The  vibrations  from  two -crank  high-speed  engines, 
even  if  mounted  on  heavy  foundations,  are  sometimes  trans- 
mitted to  considerable  distances,  especially  if  the  soil  is  moist, 
and  may  cause  considerable  annoyance  to  residents  during  the 
night  time.  The  vibrations  set  up  by  a  slow-speed  engine,  as 
shown  by  a  vibration  recording  instrument,  may  be  greater  than 
those  of  a  high-speed  engine,  but  the  period  of  vibration  being 
slower  the  annoyance  caused  is  not  so  great. 

Both  the  Belliss  and  Browett  engines  are  forced  lubrication 
engines — i.e.,  the  lubricant  is  forced  into  the  bearings  under 
pressure,  a  pump  driven  off  the  crank  shaft  being  provided  for 
the  purpose.  The  powers  and  speeds  of  these  engines  are 
approximately  the  same  as  those  of  the  Willans  engine  already 
given.  It  is  owing  to  the  short  stroke  of  these  engines  and  to 
the  system  of  forced  lubrication  that  they  are  able  to  run  at 
such  high  rotative  speeds. 

A  few  words  as  to  the  merits  of  each  type  of  engine,  and  as  to 
the  disadvantages  connected  with  its  use,  may  now  be  said. 

Slow-speed  Horizontal  Engines. — These  engines  are  very 
largely  used  in  mills  and  works  of  all  kinds  where  fair  economy, 
freedom  from  trouble  and  breakdown,  and  a  long  life  are  the 
most  important  considerations.  The  engines  usually  found  in 
mills  are  of  the  compound  type.  A  compound  engine  is  not 
quite  so  economical  as  one  of  the  triple-expansion  form,  but  the 
former  works  with  very  fair  economy.  The  consumption  of  dry, 
but  not  superheated,  steam  when  condensing  is  about  14J  Ibs. 
per  I.H.P.  per  hour.  The  wear  and  tear  of  such  engines,  if 
well  made  and  properly  lubricated,  is  very  small,  and  the  engine 
requires  but  little  attention. 

The  fact  of  the  heavy  pistons  rubbing  on  the  lower  half  of  the 


112  MECHANICAL    ENGINEERING   FOR    BEGINNERS. 

cylinder  is  not  a  good  feature,  and  for  many  years  past  it  has 
been  the  custom  to  prolong  the  piston-rod,  so  as  to  pass  through 
the  back  cover,  and  provide  it  with  a  block  running  on  a  slide, 
the  idea  being  to  carry  the  weight  of  the  piston.  A  circular 
rod,  however,  is  almost  the  worst  form  that  a  beam  designed  to 
carry  a  weight  can  take,  and  it  is  now  considered  by  many 
engineers  that  it  is  better  to  make  the  piston  of  great  width,  so 
as  to  provide  ample  bearing  surface,  and  to  dispense  with  the 
tail  rod.  It  is  claimed  that  the  wear  of  a  cylinder  having  a 
wide  and  light  piston  is  more  even,  and  less  in  extent,  than  is 
the  case  with  a  cylinder  having  a  narrow  piston  supported  by 
a  tail-rod  which  sags.  In  any  case  a  cylinder,  the  liner  of  which 
is  made  of  good  hard  cast  iron,  and  has  been  truly  bored  in  the 
first  place,  will  last  for  a  great  many  years  without  requiring  to 
be  re-bored. 

The  reason  why  horizontal  engines  are  not  employed  in  large 
electric  generating  stations  is  because  they  cannot  run  at  speeds 
sufficiently  high  to  enable  them  to  be  directly  coupled  to  the 
dynamos,  and  the  space  required  for  driving  the  latter  by  ropes 
cannot  be  given.  In  any  case  the  use  of  ropes,  involving  a  loss 
of  5  per  cent,  or  7  per  cent,  would  not  be  tolerated. 

Vertical  Slow-speed  Engines. — These  are  employed  to  the 
exclusion  of  almost  all  other  types  of  reciprocating  engine  for 
marine  work.  They  occupy  less  floor  space  than  a  horizontal 
engine,  but  are  more  expensive  to  construct.  The  frame  for 
carrying  the  cylinders  requires  to  be  much  heavier  and  stronger 
than  is  necessary  in  the  case  of  a  horizontal  engine,  the  cylinders 
of  which  in  land  work  rest  on  concrete.  Platforms  and  ladders, 
too,  are  required  with  vertical  engines,  thus  adding  to  the 
expense. 

A  good  feature  about  vertical  engines  is  that  there  is  not  the 
same  risk  of  the  cylinders  wearing  oval  as  in  a  horizontal  engine, 
as  the  piston  does  not  bear  upon  the  cylinder  walls.  A  vertical 
engine  lends  itself  more  readily  to  the  triple-expansion  form 
than  a  horizontal  engine,  and  in  some  mills  and  works  where 
first  cost  is  of  less  importance  than  extreme  economy  of  steam, 
vertical  triple -expansion  engines  may  be  found.  Vertical 
engines  of  the  marine  type  are  used  in  some  electric  generating 
stations,  but  their  comparatively  slow  speed  necessitates  a  very 
large  and  costly  generator  if  the  latter  is  to  be  coupled  direct  to 
the  engine. 

High-speed  Single-acting  Engines. — The  Willans  central- 
valve  single-acting  engine  was  introduced  at  the  time  when 
electric  lighting  was  first  being  carried  out  upon  a  considerable 
scale,  and  when  it  was  found  necessary  to  install  as  much  power 


THE    STEAM    ENGINE.  113 

as  possible  in  a  small  space.  The  high  speed  of  the  Willans 
engine  enabled  it  to  be  coupled  direct  to  the  dynamo ;  thus  the 
space  which  would  otherwise  have  been  required  for  a  belt  or 
rope  drive  was  saved,  also  the  loss  due  to  this  method  of  trans- 
mission was  avoided.  In  addition,  the  engine  was  remarkably 
economical  in  steam,  and  was  capable  of  making  very  long  runs 
without  a  stop.  It  is  on  record  that  one  of  these  engines  ran  for 
seven  months  night  and  day  in  a  copper  depositing  works  without 
a  single  stop.  The  success  of  the  Willans  engine  in  important 
generating  stations,  such  as  those  of  the  St.  James  and  Pall  Mall 
Company,  and  of  the  Westminster  Company,  led  to  its  being 
widely  adopted  all  over  the  country,  even  in  places  where  ample 
space  was  available,  and  for  many  years  this  engine  practically 
held  the  field  for  electric  lighting  and  traction  work.  In  addition 
to  the  advantages  of  high  speed  and  economy,  one  other  must  be 
referred  to.  The  engine  is  single-acting,  and,  owing  to  the 
action  of  the  air  buffers  (already  referred  to),  the  constant  thrust 
principle  is  really  carried  out.  As  there  is  no  push-and-pull 
action,  wear  of  the  brasses  does  not  give  rise  to  a  knock,  and, 
within  limits,  is  unimportant.  The  engine,  therefore,  does  not 
require  frequently  to  be  laid  off,  and  a  mechanic's  time  occupied 
in  setting  up  and  adjusting  brasses. 

The  Willans  engine  has  been  employed  in  a  few  cases  for 
driving  cotton  and  flax  mills,  for  which  purpose  the  large 
number  of  impulses  per  minute  and  consequent  even  turning 
render  it  very  suitable.  It  is  stated  that  the  output  of  a  flax 
mill  in  Ireland  was  increased  to  the  extent  of  5  per  cent, 
through  freedom  from  breakages  of  thread,  by  the  substi- 
tution of  a  Willans  three-crank  engine  for  one  of  the  slow-speed 

The  disadvantages  of  a  single-acting  engine  are  as  follows : — 
The  piston  must  have  an  area  twice  as  great  as  that  required  in 
a  double-acting  engine  of  equal  power,  running  at  the  same 
speed.  If  a  knock  is  heard  in  a  single-acting  engine,  such  as 
the  "  Willans,"  it  is  advisable  to  take  the  engine  down  at  once 
to  ascertdn  the  cause,  for  in  the  Willans  engine  the  pistons  are 
mounted  on  cast-iron  trunks,  through  which  ports  are  cut,  and 
through  which  steam  is  admitted  to  the  cylinders.  If  a  broken 
piston  ring  or  valve  ring  should  get  into  one  of  these  ports  while 
the  engine  is  running,  it  might  cause  a  very  serious  breakdown. 
As  a  fact,  however,  breakdowns  of  this  nature  are  comparatively 
rare. 

In  the  Willans  engine,  too,  there  is  a  constant,  though  small, 
loss  due  to  compressing  and  expanding  the  air  in  the  air 
chambers.  This  loss  prevents  quite  such  a  high  mechanical 

8 


114  MECHANICAL    ENGINEERING    FOR   BEGINNERS. 

efficiency  being  obtained,  as  in  a  double-acting  engine,  in  which 
there  are  no  air  buffers.* 

Double-acting  High-speed  Engines. — The  success  of  the 
Willans  single-acting  engine  incited  makers  of  double-acting 
engines  to  construct  them  to  run  at  speeds  approximating  to 
those  of  the  former,  and  by  extremely  good  workmanship  and 
by  the  use  of  forced  lubrication,  they  succeeded.  The  best 
known  high-speed  double-acting  engines  are  those  made  by 
Messrs.  Belliss  and  by  Messrs.  Browett  &  Lindley. 

The  advantages  of  these  engines  for  driving  dynamos  are 
those  possessed  by  the  Willans  engine,  and  the  engines,  con- 
forming more  nearly  as  they  do  to  the  ordinary  type  of  slow 
speed  engines,  can  be  overhauled  by  men  who  have  had  no 
special  training.  The  solid  steel  piston-rod  in  these  engines  is 
preferred  by  many  to  the  cast-iron  trunks  in  the  Willans  engine. 

For  electric  generating  purposes  a  high-speed  engine  has  in 
the  past  been  a  necessity,  but  its  place  is  seriously  threatened 
by  the  advent  of  the  steam  turbine.  For  mill  work,  and  in 
cases  where  high  speed  is  not  a  necessity,  engines  of  the  long- 
stroke  slow-running  type  are  likely  to  hold  their  own  against 
their  lighter  and  quicker  running  rivals. 

Steam  Jacketing. — The  practical  advantages  or  otherwise  of 
jacketing  steam  cylinders — i.e.,  keeping  them  surrounded  by  live 
steam — has  been  under  discussion  for  a  great  many  years  past. 
It  is  claimed  that  by  jacketing  a  cylinder  and  imparting  heat  to 
its  walls  while  expansion  is  taking  place,  and  during  exhaust, 
the  fall  in  temperature  of  the  walls  is  reduced  and  initial  con- 
densation is  lessened.  On  the  other  hand,  a  certain  proportion 
of  the  steam  in  the  jacket  is  condensed,  and  opponents  of  jacket- 
ing say  that  as  much  steam  is  lost  in  the  jacket  as  is  saved  in  the 
cylinder. 

Experiments  appear  to  show  that   there  is  a  gain   through 

*  If  this  little  book  should  fall  into  the  hands  of  a  station  engineer,  who 
would  like  to  ascertain  the  loss  of  efficiency  due  to  the  air  buffers,  and  who 
has  been  unable  to  ascertain  the  loss  by  means  of  an  indicator  card,  he  can 
easily  do  so,  assuming  the  engine  is  coupled  to  a  direct  current  dynamo, 
by  the  following  method  : — The  first  time  the  engine  is  down  for  any 
purpose,  remove  the  steam  cylinders,  pistons,  and  high-pressure  trunks, 
leaving  the  guide  piston,  low-pressure  trunks,  and  air  buffer  covers  in 
position.  Run  the  dynamo  as  a  motor,  and  note  the  current  required  to 
drive  the  crank  shaft  at  its  full  speed,  with  the  guide  pistons  and  low- 
pressure  trunk  running — i.e.,  with  the  buffers  in  action.  Then  remove  the 
guide  pistons,  and  note  what  current  is  required  to  run  the  crank  shaft  at 
full  speed — i.e.,  with  the  buffers  out  of  action.  The  difference  of  current 
(neglecting  the  small  difference  of  dynamo  efficiency)  will  show  the  loss 
due  to  compressing  the  air  in  the  buffers.  The  loss  is  not  truly  shown  by 
an  indicator  card. 


THE    STEAM    ENGINE.  115 

jacketing,  if  properly  carried  out.  This  gain  is  probably  due  to 
the  fact  that  it  is  better  to  condense  the  steam  outside  the 
cylinder  than  inside  it,  as  it  is  the  film  of  water  inside  the 
cylinder  walls  which  enables  the  transfer  of  heat  to  take  place 
so  easily,  and  which  robs  them  of  so  much  heat  when  re-evapor- 
ation takes  place.  Whether  the  gain  in  actual  practice  is  as 
great  as  in  experimental  trials  is,  however,  open  to  doubt,  for  it 
must  not  be  overlooked  that  for  the  jacket  to  be  effective  the 
condensed  steam  must  be  got  rid  of,  and  those  who  have  had 
some  experience  in  connection  with  steam  traps  know  that  they 
have  a  tendency  either  to  stick,  or  to  leak.  If  the  trap  sticks 
the  advantage  of  the  jacket  is  lost,  and  if  the  trap  leaks  it  is 
quite  possible  that  the  steam  lost  in  this  way  may  equal  that 
saved  in  the  cylinder. 

There  is,  however,  one  advantage  in  jacketing  an  engine 
which  the  maker  cannot  afford  to  overlook — it  makes  a  good 
selling  point.  If  A's  representative  can  say  to  an  intending 
purchaser,  "My  engine  is  jacketed,  while  B's  is  not,"  it  may 
possibly  turn  the  scale  in  A's  favour ;  and  it  must  be  remem- 
bered that,  distasteful  as  the  fact  may  appear,  the  majority  of 
engine  builders  are  in  business,  not  for  the  pleasure  of  the  thing, 
nor  for  the  purpose  of  contributing  papers  to  learned  societies, 
but  to  make  money  for  themselves,  or  for  their  shareholders. 

Calculating  the  Power  of  Compound  Engines. — The 
reader  has  seen  how  to  work  out  the  power  of  a  single  cylinder, 
or  simple  engine ;  to  calculate  the  horse-power  of  a  compound  or 
triple-expansion  engine,  the  formula  for  the  simple  engine  holds 
good ;  but  if  the  student  has  not  got  the  actual  indicator  dia- 
grams before  him,  care  must  be  taken  to  estimate  the  mean 
pressures  rightly.  With  condensing  engines  it  is  also  more 
convenient  to  make  the  calculation  with  absolute  pressures — i.e., 
including  the  atmosphere.  Instead  of  saying  75  Ibs.  above  the 
atmosphere  we  should  say  89-7  Ibs.  absolute,  or,  roughly,  90  Ibs. 
absolute. 

We  will  now  take  the  case  of  a  double-acting  compound  con- 
densing engine,  having  one  H.P.  cylinder  10  inches  diameter 
and  one  L.P.  cylinder  18  inches  diameter,  the  stroke  of  each  8 
inches.  The  speed,  300  revolutions  per  minute.  In  working 
out  the  power  of  the  simple  engine  with  approximately  90  Ibs. 
absolute  pressure,  we  assumed  the  mean  pressure  to  be  about 
65  Ibs.  absolute.  If,  however,  instead  of  passing  the  exhaust 
steam  direct  to  the  atmosphere,  we  take  it  to  another  and  larger 
cylinder  to  do  more  work,  the  pressure  of  the  steam  acting  on 
the  large  L.P.  piston  will  also  exert  a  back  pressure  upon  the 
small  H.P.  piston,  so  that,  instead  of  having  a  mean  effective 


116  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

pressure  of  65  Ibs.  absolute  on  the  H.P.  piston,  we  shall  only 
have  an  effective  mean  pressure  of  about  40  Ibs.  The  mean 
pressure  in  the  L.P.  cylinder  will,  of  course,  be  much  less, 
as  its  area  is  about  3J  times  greater  than  that  of  the  H.P. 
cylinder,  and  it  only  receives  its  steam  after  this  leaves  the  H.P. 
cylinder.  If  in  this  case  we  cut  off  at  '65  of  the  stroke,  we  shall 
expand  the  steam  about  5  times  altogether.  The  effective  mean 
pressure  we  might  get  in  the  L.P.  cylinder  would  be  about  20 
Ibs.  absolute.  We  should,  therefore,  have  to  take  the  area  of  the 
10-inch  piston — viz.,  78 '5  inches — as  having  an  effective  pres- 
sure of  40  Ibs.,  and  the  area  of  the  18-inch  piston — viz., 
254-4  inches— as  having  an  effective  pressure  of  20  Ibs.  The 
result  would  be— 78-5  x  40  =  3,140  Ibs.,  and  254-4x20  =  5,088 
Ibs.,  or  8,228  Ibs.  total  effective  pressure  on  the  two  pistons. 
For  the  sake  of  simplicity  in  the  calculations,  it  is  usual  to  refer 
this  total  pressure  to  a  corresponding  pressure  on  the  L.P.  piston. 
In  this  case  the  area  of  the  L.P.  piston  is  254-4  inches ;  and  if 
we  divide  the  total  effective  pressure  on  the  two  pistons  by  this 
area,  we  shall  have  the  corresponding  effective  pressure  on  the 
L.P.  piston.  Thus,  8,228  4-  254'4  =  32-3  Ibs.  mean  pressure,  so 
that  in  our  calculations  for  horse-power  we  may  ignore  the  H.P. 
piston,  and  take  into  account  the  L.P.  piston  only,  with  the 
corrected  mean  pressure  referred  to  it. 

OQ~R    A  T> 

The  formula  for  horse-power,  as  stated  before,  is „»  ~^ 

The  calculation  for  the  compound  engine  will,  therefore,  be  — 
2  x  -666  x  300  x  254-4  x  32-3 
33,000 

Testing  Steam  Engines. — There  are  two  methods  of  ascer- 
taining how  much  steam  an  engine  uses.  The  first,  by  weighing 
the  water  pumped  into  the  boiler,  and  the  second,  by  weighing 
the  steam  (condensed)  as  it  leaves  the  engine.  The  first  method 
is  usually  adopted  when  it  is  desired  to  make  a  test  after  an 
engine  has  been  fixed  at  the  site  and  is  doing  actual  work  ;  but, 
unless  precautions  are  taken,  this  method  frequently  makes  the 
engine  appear  to  use  more  steam  than  is  really  the  case.  For 
instance,  should  there  be  a  small  leak  in  the  furnace  of  the 
boiler,  any  water  which  escapes  is  immediately  turned  into 
steam,  and  probably  passes  away  with  the  furnace  gases 
unnoticed. 

To  avoid  such  a  loss,  it  is  advisable  to  make  a  "  still "  test  of 
the  boiler  over  a  period  of  10  or  12  hours.  For  this  test,  steam 
is  raised  in  the  boiler  to  within  5  or  6  Ibs.  of  the  pressure  at 
which  the  safety  valves  blow  off.  The  level  of  the  water  in  the 


THE    STEAM    ENGINE.  117 

boiler  is  carefully  observed,  all  valves  are  then  closed  (the  steam 
pipe  should  be  closed  with  a  blank  flange),  and  steam  maintained 
at  a  pressure  below  blow-off  pressure.  At  the  end  of  10  or 
12  hours,  water  which  has  been  weighed  is  pumped  into  the 
boiler  until  the  original  level  is  reached.  This  weight  of 
water  is  the  amount  of  boiler  leakage  during  the  10  or  12 
hours'  test.  The  leakage  is  often  considerable,  even  in  a 
boiler  which  has  appeared  to  be  tight  when  tested  under 
hydraulic  pressure. 

In  carrying  out  an  engine  trial  by  this  method,  care  must  be 
taken  to  see  that  during  the  trial  the  boiler  safety  valve  does 
not  blow  off,  that  there  are  no  leaks  of  steam  from  the  steam 
pipe  connecting  the  boiler  to  the  engine,  and  thafc  no  auxiliary 
machinery,  such  as  a  separate  donkey  feed  pump,  is  supplied 
from  the  boiler  into  which  water  is  being  weighed. 

The  second  method  is  more  accurate  so  far  as  the  engine  is 
concerned,  and  is  that  usually  adopted  at  the  works  of  engine- 
makers  who  are  in  the  habit  of  testing  their  engines  before 
delivery.  By  this  method  the  steam  as  it  leaves  the  engine  is 
condensed  and  passed  into  a  tank,  which  is  placed  on  a  weighing 
machine.  It  is  certain  that  all  steam  which  goes  into  an  engine 
must  come  out  of  it,  otherwise  the  engine  would  become  choked 
with  water. 

If  the  engine  is  ultimately  to  work  non-condensing,  and  non- 
condensing  results  are  required,  the  chamber  in  which  the 
steam  is  condensed  is  left  open  to  the  atmosphere,  so  that  there 
is  no  vacuum.  By  this  system  of  measurement  boiler  leaks  are 
immaterial,  as  the  engine  is  only  debited  with  the  exhaust  steam 
which  comes  out  of  it.  The  boiler,  too,  may  drive  any  auxiliary 
machinery  while  the  test  is  being  made. 

A  test  of  one  or  two  hours'  duration  made  in  this  way  is  more 
accurate  than  a  10  or  12  hours'  test  made  by  the  method  first 
described,  as  readings  can  be  taken  every  few  minutes,  and  if 
the  readings  tally  with  one  another,  the  result  may  be  relied 
upon  implicitly. 

To  take  an  actual  case.  Suppose  an  engine  which  is  giving 
100  horse-power  passes  regularly  every  minute  25  Ibs.  of  water 
into  the  tank,  we  know  that  the  engine  is  using  1,500  Ibs.  of 
steam  per  hour.  If  now  we  divide  this  total  water  by  the 
horse-power,  we  see  that  the  engine  is  using  15  Ibs.  of  steam  per 
horse-power  per  hour.  It  is  not  necessary  to  have  a  tank  of 
very  large  proportions,  for,  if  the  readings  tally,  it  does  not 
matter  how  often  the  tank  is  emptied.  The  arm  of  the  weigh- 
bridge upon  which  the  tank  is  placed  is  usually  arranged  so  that 
as  soon  as  a  given  quantity  of  water  has  passed  from  the  engine 


118  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

into  the  tank  the  arm  rises,  the  observer  notes  the  time  and 
pushes  forward  a  weight  which  depresses  the  arm ;  as  soon  as 
the  given  quantity  of  water  has  again  entered  the  tank,  the  arm 
again  rises  and  the  observer  again  notes  the  time,  pushes  forward 
the  weight,  and  so  on. 

Having  got  the  boiler,  engine,  and  weighing  apparatus,  how  is 
a  load  to  be  provided  for  the  engine,  assuming  we  wish  to  test  it 
before  it  leaves  the  maker's  works  1 

If  the  engine  is  coupled  direct  to  a  dynamo  the  matter  is 
comparatively  easy;  all  that  is  necessary  is  to  provide  resistances 
in  the  form  of  wires  through  which  the  current  will  have  to 
travel ;  the  power  of  the  engine  will  be  absorbed  in  generating 
the  electric  current,  and  the  current  will  be  dissipated  in  the 
form  of  heat  in  passing  through  the  wires  of  low  conductivity. 
If  suitable  wire  resistances  are  not  available,  a  resistance  can  be 
formed  by  immersing  two  metal  plates  in  a  tank  of  water,  and 
making  the  current  force  its  way  through  the  water  between 
them.  Such  a  water  resistance  is,  however,  not  so  satisfactory 
as  a  wire  resistance,  as  it  is  difficult  to  keep  the  load  steady. 
The  electrical  method  of  providing  a  load  for  the  engine  is  a 
good  one,  as  if  the  volts  and  amperes  given  by  the  dynamo  are 
noted  and  its  efficiency  is  known,  the  actual  brake  horse-power 
given  by  the  engine  can  be  calculated.  Chapter  xi.  will  show 
how  such  calculations  are  made.  If,  however,  the  engine  is  not 
coupled  to  a  dynamo,  a  load  can  be  put  on  the  engine  by  means 
of  a  brake. 

This  brake,  too,  can  be  made  to  show  how  much  horse-power 
the  engine  is  giving.  In  this  way ;  we  know  that  1  horse- 
power is  the  equivalent  of  lifting  33,000  Ibs.  1  foot  high  in  one 
minute.  Now,  if  we  were  to  arrange  matters  so  that  the 
friction  caused  by  a  band  surrounding  a  pulley  which  was 
travelling  at  the  rate  of  1  foot  in  one  minute,  exerted  a  pull 
on  the  band  of  33,000  Ibs.,  this  would  be  equivalent  to  lifting 
33,000  Ibs.  1  foot  in  one  minute.  A  rope-brake  working  on 
this  principle  is  frequently  used.  It  consists  of  several  pieces 
of  rope  arranged  to  form  a  flat  band  (similar  to  the  plaited 
leather  horse -girths  sometimes  used  when  schooling  young 
horses).  The  rope  is  well  greased,  and  is  lapped  round  the 
pulley  of  the  engine  to  be  tested,  one  end  is  attached  to 
a  portable  weighing  machine  of  the  "  Denison  "  type,  preferably 
carried  by  a  crane,  the  other  is  attached  to  a  strong  spring 
balance.  Of  course,  in  actual  practice  the  face  of  the  pulley 
travels  much  faster  than  1  foot  in  one  minute,  and  the  pull  on 
the  ropes  is  much  less  than  33,000  Ibs.  for  every  horse-power, 
but  these  figures  make  the  principle  clear. 


THE    STEAM    ENGINE.  119 

In  the  case  of  a  brake  load,  the  work  done  by  the  engine  is 
absorbed  in  friction,  and  passes  away  in  the  form  of  heat.  The 
fact  that  the  work  done  by  the  engine  passes  away  as  heat,  does 
not  seem  to  be  clearly  realised  by  all  of  those  who  have  to  put  a 
load  on  an  engine  by  means  of  a  brake.  The  author  knew  one 
individual  who  felt  distinctly  aggrieved  because  the  ropes  forming 
the  brake  got  too  hot  and  charred,  in  spite  of  his  having  used 
tallow,  &c.  He  complained  that  he  had  to  throw  buckets  of 
water  on  the  brake  ropes  and  pulley  to  keep  them  cool,  thus 
causing  a  great  mess  on  the  floor. 

In  case  the  reader  would  like  to  see  approximately  how  much 
water  would  be  required  to  be  thrown  on  a  rope-brake  absorbing 
100  H.P.,  in  order  to  keep  it  cool,  neglecting  the  dissipation  of 
heat  by  radiation  and  conduction,  the  method  of  making  the 
calculation  is  explained.  It  is  as  follows : — 100  H.P.  x  33,000 
ft.-lbs.  =  3,300,000  ft.-lbs.  of  energy  to  be  got  rid  of  in  one 
minute.  We  read  in  Chapter  in.  that  772  ft.-lbs.  are  the 
equivalent  of  1  B.T.U.,  also  that  1  B.T.U.  will  raise  1  Ib.  of 
water  by  1°  F.,  so  that,  if  we  divide  3,300,000  by  772,  we  shall 
see  how  many  thermal  units  have  to  be  got  rid  of,  and  how 
much  water  will  be  required.  3,300,000  +  772  =  4,274  B.T.U. 
Assuming  the  water  in  the  buckets  is  at  a  temperature  of  60°  F., 
and  after  being  thrown  on  the  brake  its  temperature  is  raised  to 
212°F.,  each  Ib.  of  water  will  take  away  152  thermal  units,  and 
a  gallon  of  water  (weighing  10  Ibs.)  will  take  away  1,520  B.T.U. 
Now,  if  we  divide  4,274  by  1,520,  the  answer  is  2-81  gallons  of 
water  per  minute,  so  that  2 -81  gallons  of  water  per  minute  will 
require  to  be  thrown  on  the  brake  to  keep  it  cool  when  absorb- 
ing 100  H.P.  If,  however,  instead  of  throwing  water  on  the 
brake  by  the  bucketful,  and  only  raising  its  temperature  to  210° 
or  212°  F.,  matters  could  have  been  arranged  so  that  the  water 
was  thrown  on  the  brake  in  a  fine  spray,  say  from  a  hose  with  a 
fine  rose,  and  the  whole  of  the  water  was  turned  into  steam,  a 
very  much  smaller  quantity  of  water  would  have  been  required, 
for  we  read  in  Chapter  in.  that  a  very  much  larger  quantity  of 
heat  is  required  to  evaporate  a  Ib.  of  water  at  212°  than  is 
required  merely  to  raise  it  to  this  temperature.  Let  us  see  how 
much  water  would  be  required  to  keep  the  brake  cool  if  all  the 
water  thrown  on  to  it  were  vaporised.  It  has  already  been  said 
that  the  latent  heat  of  1  Ib.  of  steam  at  212°  F.  is  966  B.T.U., 
therefore  the  water,  if  turned  into  steam,  will  absorb  all  this 
heat  from  the  brake,  as  well  as  the  152  B.T.U.  required  to  raise 
the  water  from  60°  to  2 1 2°.  Every  pound  of  water  thrown  on  the 
brake  will,  therefore,  absorb  966  +  152,  or,  say,  1,118  B.T.U., 
and  as  we  have  seen  that  4,274  B.T.U.  must  be  got  rid  of  per 


120  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

minute,  3-82  Ibs.,  or  a  little  more  than  J  of  a  gallon  of  water  per 
minute,  will  answer  the  purpose. 

A  sound  and  working  knowledge  of  the  principles  of  heat  and 
steam  would  have  shown  the  engineer,  to  whom  we  have  referred, 
(1)  that  the  brake  was  bound  to  get  hot  when  absorbing  power; 
and  (2)  that  by  using  smaller  quantities  of  water,  and  turning 
most,  if  not  all,  of  it  into  steam,  he  could  have  prevented  a  good 
deal  of  the  mess  on  the  floor. 

It  will  be  realised,  from  the  foregoing,  that  any  form  of 
friction  brake,  unless  suitable  means  are  provided  for  taking 
away  the  heat,  is  quite  unsuitable  for  absorbing  large  powers. 
When  dealing  with  engines  of  small  power,  sufficient  heat  is 
lost  by  radiation  to  enable  the  brake  to  keep  fairly  cool. 

A  much  more  convenient  form  of  brake  is  the  Froude  water 
brake  (constructed  by  Mather  &  Platt,  of  Manchester).  In  this 
brake  there  is  an  internal  wheel,  carried  on  a  long  shaft,  having 
a  coupling  at  one  or  both  ends;  the  wheel  consists  of  two  saucer- 
like  vessels  placed  back  to  back,  each  divided  by  vanes  having  a 
forward  rake.  The  external  casing,  which  is  free  to  turn  on  the 
shaft,  has  two  similar  saucer-like  vessels,  also  divided  by  vanes 
having  a  backward  rake.  This  external  casing  carries  a  long 
arm  or  lever,  at  the  end  of  which  weights  are  suspended.  The 
engine  which  is  to  be  tested  is  coupled  to  the  flange  on  the  shaft 
carrying  the  wheel,  and  water  is  admitted  to  the  brake.  The 
rotation  of  the  wheel  acting  on  the  water  tends  to  make  the 
casing  rotate,  but  it  is  prevented  from  doing  so  by  the  weights 
carried  at  the  end  of  the  arm.  The  rotary  action,  however, 
causes  the  arm  to  rise  and  lift  the  weights,  and  the  weights 
being  known,  the  actual  power  can  be  calculated. 

The  water  passes  through  the  brake  in  a  continuous  stream, 
the  work  done  being  in  the  form  of  heat  imparted  to  the  water 

Such  a  brake  is  somewhat  expensive,  especially  if  of  large  size, 
and  is  only  possessed  by  a  few  engineering  firms,  but  its  value 
may  be  realised  from  the  following  incident  which  came  to  the 
author's  knowledge  a  short  time  ago.  A  firm  of  engine  makers 
accepted  a  contract  for  some  engines,  which  were  guaranteed  to 
give  a  certain  mechanical  efficiency.  The  engines  were  duly 
constructed,  and  coupled  to  some  dynamos,  which  were  also 
guaranteed  to  give  a  certain  mechanical  efficiency.  When  the 
plant  was  tested,  the  combined  efficiency  was  not  equal  to  that 
which  would  have  been  obtained  had  the  engine  maker's  and 
dynamo  maker's  guarantees  been  kept.  Perhaps  we  had  better 
explain  what  is  meant  by  combined  efficiency.  Let  us  say  that 
the  engine  maker  guaranteed  a  mechanical  efficiency  of  90  per 
cent,  and  the  dynamo  maker  an  efficiency  of  90  per  cent.,  the 


THE    STEAM    ENGINE. 


121 


combined  efficiency  of  the  plant  should  be  81  per  cent.  That  is 
to  say,  the  actual  electric  output  should  be  81  H.P.  for  every 
100  H.P.  developed  in  the  engine  cylinders. 


Fig.  46. — Crosby  indicator. 


Fig.  46a. — Crosby  indicator  for  superheated  steam. 

The  efficiency  in  the  case  in  question  not  being  obtained,  the 
question  arose  as  to  who  was  in  fault.  The  power  of  the  engines 
being  too  great  to  admit  of  the  use  of  a  rope  brake,  the  matter 


122  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

was  settled  by  the  purchase  of  a  Froude  brake,  when   it  was 
found  that  the  fault  lay  with  the  dynamos. 

We  will  assume  that  all  arrangements  have  now  been  made  for 
giving  the  engine  a  suitable  load ;  how  are  we  to  ascertain  exactly 
what  I.H.P.  the  engine  under  test  is  giving]  The  stroke  of  the 
engine  is  known ;  the  revolutions  per  minute  at  which  the  engine 
is  running  can  be  ascertained  by  a  mechanical  speed  counter ;  the 
area  of  the  piston  is  known ;  all  then  we  need  to  know  is  the 
mean  effective  pressure  on  the  pistons,  or  what  P  stands  for  in 
the  formula — 

2SRAP 
33,000  ' 

The  actual  mean  pressure  exerted  on  the  piston  of  an  engine  is 
found  by  means  of  an  indicator.  An  indicator  of  the  Crosby 
make  is  shown  by  Fig.  46.  This  instrument  consists  of  a  small 
barrel,  or  cylinder,  in  which  a  piston  works  against  a  spring. 
The  piston  which  in  a  good  indicator  is  nearly  frictionless,  actu- 
ates a  pencil  at  the  end  of  an  arm  or  lever.  When  the  indicator 
barrel  is  placed  in  communication  with  the  engine  cylinder,  any 
pressure  in  the  latter  causes  the  indicator  piston  to  rise,  and  the 
pencil  marks  a  piece  of  paper  or  card.  If  the  card  were  held 
stationary,  a  vertical  line  only  would  be  recorded  due  to  the 
rising  and  falling  of  the  indicator  piston,  but  by  moving  the 
card  so  that  the  portion  under  the  pencil  corresponds  with  the 
position  of  the  engine  piston,  a  continuous  line  or  diagram  is 
drawn  showing  the  pressure  exerted  on  the  engine  piston  at  all 
points  of  the  stroke.  The  drum  to  which  the  card  is  attached  is 
rotated  by  means  of  a  cord  attached  to  any  reciprocating  part  of 
the  engine  which  gives  the  required  motion. 

The  Crosby  indicator,  owing  to  the  lightness  of  its  working 
parts  and  to  the  accuracy  with  which  it  is  made,  is  largely  used 
for  indicating  high-speed  engines,  for  which  purpose  the  older 
patterns  of  indicator  having  heavier  parts  and  a  longer  stroke 
are  unsuitable.  The  latest  development  of  the  Crosby  indicator 
is  shown  by  Fig.  46a,  from  which  it  will  be  seen  that  the  spring 
against  which  the  indicator  piston  works  is  placed  outside  the 
barrel.  This  is  a  great  improvement,  especially  in  cases  where 
superheated  steam  is  used,  as  the  temperature  of  the  steam  does 
not  affect  the  action  of  the  spring.  Moreover,  when  the  spring 
is  placed  ouside  the  barrel,  there  is  no  possibility  of  its  causing 
the  piston  to  bear  unduly  on  one  side  of  the  barrel. 

Springs  of  different  strengths  are  supplied  with  the  indicator. 
Thus,  one  spring  will  travel  upwards  1  inch  with  a  pressure  of 
40  Ibs.  in  the  barrel,  while  another  will  travel  1  inch  upwards 


THE    STEAM    ENGINE.  123 

with  a  pressure  of  10  Ibs.  One  is  called  a  40-lb.  spring,  the 
other  a  1 0-lb.  spring.  The  spring  used  is  noted  on  the  indicator 
card  by  the  tester ;  a  much  stronger  spring  is  used  for  indicating 
the  H.P.  cylinder  than  for  the  L.P.  cylinder.  To  ascertain  the 
pressure  shown  by  a  diagram,  if  a  40-lb.  spring  had  been  used,  a 
boxwood  scale  graduated  40  Ibs.  to  the  inch  would  be  used.  If 
a  10-lb.  spring  had  been  used,  a  scale  graduated  10  Ibs.  to  the 
inch  would  be  employed. 

The  indicator  barrel  is  connected  to  the  cylinder  by  means  of 
a  short  pipe  and  cocks,  arranged  so  that  the  barrel  of  the  indi- 
cator may  be  shut  off  from  the  cylinder,  and  placed  in  communi- 
cation with  the  atmosphere ;  when  this  is  done,  if  motion  is  given 
to  the  card,  a  straight  horizontal  line  is  drawn,  which  is,  of 
course,  the  atmospheric  line. 

Optical  Indicators. — For  speeds  above  500  or  600  revs, 
per  minute,  an  indicator  with  a  piston,  arm,  and  pencil,  is 
unsuitable  owing  to  the  inertia  of  the  moving  parts.  To  indicate 
petrol  or  other  engines  running  at  very  high  speeds,  optical 
indicators  are  used.  In  these  the  piston,  lever,  and  pencil  are 
replaced  by  a  diaphragm  and  mirror.  The  movement  of  the 
diaphragm  deflects  the  mirror,  and  a  spot  of  light  is  thus 
thrown  on  a  moving  screen.  With  a  fast-running  engine,  the 
spot  of  light  traces  what  is  apparently  a  continuous  line,  so 
that  the  shape  of  the  diagram  is  seen  while  the  engine  is 
running.  To  obtain  a  record  of  the  diagram,  photography  is 
resorted  to. 

Figs.  47  and  48  show  two  indicator  diagrams  taken  respectively 
from  the  H.P.  and  L.P.  cylinders  of  a  1,200  I.H.P.  compound 
condensing  Corliss  engine  supplied  for  driving  a  mill.  At  the 
time  the  diagrams  were  taken  the  engine  was  called  upon  to 
give  between  f  and  f  of  its  full  power  only,  but  as  the  dia- 
grams are  reproduced  from  those  of  large  size  and  are  taken 
from  a  good  engine,  they  will  serve  to  show  the  use  of  such 
diagrams,  and  how  the  power  of  an  engine  can  be  worked  out 
from  them. 

The  explanation  of  the  diagrams  is  briefly  thus — The  upper 
line  represents  the  pressure  of  the  steam  in  the  cylinder  on  the 
outward  stroke,  the  lower  line  the  pressure  on  the  return  stroke. 
When  steam  is  first  admitted  to  the  cylinder  the  indicator  pencil 
rushes  up  to  A;  this  initial  pressure,  in  the  diagram  before  us, 
is  well  maintained  until  the  point  of  cut-off,  when  the  steam 
begins  to  expand  and  the  pressure  falls.  Point  B  shows  approxi- 
mately where  the  exhaust  port  begins  to  open ;  it  is  fairly  wide 
open  at  C,  and  continues  to  remain  open  until  D  is  reached 
when  the  exhaust  port  closes,  the  steam  remaining  in  the 


124 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


cylinder  begins  to  be  compressed,  the  pressure  rising  to  E.  At 
this  point  the  steam  port  again  opens,  and  the  pencil  rushes  up 
to  A. 


Cut  off 


Fig.  47. — Indicator  diagram. 

The  card  taken  from  the  other  end  of  the  cylinder  was 
practically  identical,  but  is  not  shown  in  order  to  avoid 
confusion. 


LP 


Zero  Line 


Fig.  48. — Indicator  diagram. 


THE    STEAM    ENGINE.  125 

The  diagram  shown  by  Fig.  47  was  taken  with  a  40-lb  spring, 
so  that  every  inch  of  height  represented  40  Ibs.  pressure  on  the 
piston.  The  diagram  has  been  reduced  in  reproduction,  but 
a  scale  is  given  at  the  side  so  that  the  reader  can  measure 
it  for  himself.  To  obtain  the  mean  pressure  shown  by  the 
diagram,  an  engine  maker  would  use  an  instrument  called  a 
planimeter.  By  means  of  this  instrument  the  area  of  any 
irregular  space  can  be  measured  by  merely  running  the  pointer 
round  the  boundary  line,  when  the  area  can  be  read  off  on 
the  index;  if,  knowing  the  area,  we  divide  it  by  the  length 
of  the  diagram  we  get  the  mean  height.  However,  as  a  plani- 
meter is  not  possessed  by  every  one,  the  next  best  way  to  find 
the  mean  pressure  shown  by  the  diagram  is  to  divide  it  by  10 
vertical  lines,  or  ordinates,  equally  spaced.  Then  read  off  the 
height  (or  pressure  in  pounds)  of  each  by  the  scale,  add  them  all 
together,  and  divide  by  the  number  of  lines.  This  will  give  the 
average  or  mean  pressure. 

The  particulars  of  the  engine  from  which  the  diagrams  were 
taken  are  given  below.  The  pressure  shown  by  the  boiler  gauge 
was  112  Ibs.  above  atmosphere  or  126*7  absolute. 

Stroke, 5     feet. 

Revolutions, 60     per  minute. 

Diameter  of  H.  P.  piston,      ....  30     inches. 
Area  of  H.P.  piston,  after  deducting  area  of 

6-inch  piston-rod  and  tail-rod,       .         .  678*5      ,, 

Diameter  of  L. P.  piston,       .         .         .         .  56          ,, 
Area  of  L.P.  piston  after  deducting  area  of 

piston-rod  and  tail-rod,           .         .         .  2,434-7       ,, 

Ratio  of  H.P.  to  L.P.  cylinder,    .        .        .  1  to 3' 59 

The  mean  pressures  shown  by  the  vertical  lines  are : — 

H.P.  diagram.  L.P.  diagram. 

76  1 1  *5 

89  16 

68  15-75 

44  13-25 
30  9-5 

22  7*5 

15  6 

12  5 

8  4 

6  2*5 

370  91 

If  we  divide  the  above  totals  by  the  number  of  lines — viz.,  10 — 
we  get  an  average  effective  pressure  of  37  Ibs.  per  square  inch  on 
the  H.P.  piston,  and  9-1  Ibs.  on  the  L.P.  piston,  or  an  equivalent 


126  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

pressure  of  19 '4*  Ibs.  on  the  L.P.  piston.  We  do  not  need  to 
take  any  account  of  the  back  pressure  against  the  pistons,  as  we 
have  measured  the  height  of  the  vertical  lines  not  from  the 
atmospheric  line,  or  from  the  zero  line,  but  from  the  line 
forming  the  bottom  of  the  diagrams,  which  shows  the  pressure 
at  which  the  exhaust  steam  leaves  the  H.P.  cylinder  to  enter 
the  receiver,  and  the  pressure  at  which  the  exhaust  steam  leaves 
the  L.P.  cylinder  to  enter  the  condenser.  Having  obtained  the 
mean  pressure  from  the  diagrams,  we  can  now  calculate  the 
horse-power  by  the  formula  already  given ;  the  figures  are — 

2x5  x  60  x  2,434-7  x  194 

33,000  '  LILR' 

so  that  the  engine  was  indicating  860  I. H.P.  when  the  cards 
were  taken.  The  H.P.  diagram  enables  us  to  see  what  drop  in 
pressure  there  was  between  the  boiler  and  the  cylinder.  The 
boiler  pressure  was  1 1 2  Ibs.  above  atmosphere,  and  the  card  or 
diagram  shows  that  the  highest  pressure  in  the  cylinder  was 
103  Ibs.  above  atmosphere,  or  a  drop  of  9  Ibs.  The  lower  line 
of  the  diagram  shows  the  pressure  at  which  the  steam  leaves  the 
H.P.  cylinder;  this  pressure  varies  from  7  Ibs.  to  12  Ibs.  above 
atmosphere,  say  9 J  Ibs.  average  pressure ;  the  top  line  of  the 
L.P.  diagram  shows  that  the  highest  pressure  of  the  steam 
received  in  the  L.P.  cylinder  was  7*5  Ibs.  The  drop,  which  is 
not  excessive,  is  caused  by  the  steam  having  to  pass  through  the 
exhaust  port  and  receiver,  and  enter  through  the  steam  port  of 
the  L.P.  cylinder. 

The  consumption  of  steam  in  the  engine  was  only  14-51  Ibs. 
per  I. H.P.  per  hour;  an  extremely  good  result  for  a  compound 
engine  working  at  less  than  ^  load.  It  was  not  possible  to 
measure  the  actual  load  on  the  engine,  so  the  mechanical 
efficiency  or  consumption  per  brake  horse-power  is  not  known 
as  well  as  per  indicated  horse-power. 

When  it  is  not  possible  to  measure  the  actual  load,  the  ap- 
proximate efficiency  can  be  ascertained  by  running  the  engine 
light  and  taking  diagrams  ;  these  will  show  the  amount  of  steam 
required  to  turn  the  engine  round.  This  rough  efficiency  test 
must  be  carried  out  before  the  driving  ropes  or  belts  are  in 
position.  Assuming  the  mean  pressure  required  when  the 
engine  is  fully  loaded  is  40  Ibs.,  and  the  diagram  shows  a 
mean  pressure  of  4  Ibs.  when  running  the  engine  light,  the 
engine  efficiency  is  90  per  cent.  This  method  is,  however,  not 

*  The  L.P.  piston  is  3'59  times  greater  than  the  H.P.  piston;  a  mean 
pressure  of  37  Ibs.  on  the  H.P.  piston  is  therefore  equivalent  to,  say,  10'3 
Ibs.  on  the  L.P.  piston— 10 '3  +  9'1  -  19 -4. 


THE    STEAM    ENGINE.  127 

very  reliable,  as,  in  the  first  place,  diagrams  showing  such  a 
small  mean  pressure  divided  over  the  two  cylinders  are  not 
very  accurate,  and,  in  the  second  place,  the  friction  of  the 
bearings  and  guides  is  greater  at  full  than  at  light  loads.  It 
has  been  found  that  with  high-speed  engines  constructed  on 
interchangeable  lines,  and  with  the  most  accurate  workmanship, 
the  mean  pressure  required  to  run  them  light  is  considerably 
higher,  and  the  consumption  of  steam  greater,  when  first  started 
than  it  is  after  the  engine  has  had  a  few  days'  running;  by 
which  time  the  working  parts  will  have  acquired  a  fair  working 
face. 

When  indicating  a  Willans'  single-acting  engine,  an  indicator 
card  is  always  taken  from  the  receiver  beneath  the  H.P.  cylinder 
(also  beneath  the  intermediate  cylinder,  if  there  is  one),  and  the 
mean  pressure  shown  by  such  card  is  added  to  the  mean  pressure 
acting  upon  the  upper  side  of  the  piston.  The  reason  for  this 
is  as  follows  : — The  lower  line  of  the  H.P.  diagram  shows  the 
pressure  of  steam  above  the  piston  while  the  cylinder  is  ex- 
hausting to  the  receiver  and  the  piston  is  on  its  up  stroke ;  as 
soon  as  the  piston  reaches  the  top  of  its  stroke  the  exhaust  port 
closes.  On  the  down  stroke  the  diagram  shows  the  pressure  of 
steam  driving  the  piston  down,  but  it  does  not  show  what  is 
going  on  beneath  the  piston ;  as  a  fact,  the  pressure  beneath  the 
piston — i.e.,  in  the  receiver — begins  to  fall  as  soon  as  the  L.P. 
cylinder  commences  to  draw  steam  from  it.  This  fall  of  pressure 
is  shown  by  the  receiver  diagram,  and  is  added  to  the  pressure 
acting  above  the  piston. 


129 


CHAPTER   VII. 

THE    STEAM    ENGINE. 

(PART  II.) 

Simple  Flywheel  Calculations. — In  addition  to  being  able 
to  work  out  the  horse-power  of  an  engine,  it  is  essential  that 
a  young  engineer  should  be  able  to  make  simple  calculations 
required  in  connection  with  the  flywheel,  so  that,  for  instance, 
he  may  be  able  to  compare  the  value  of  two  flywheels  differing 
in  size  or  weight,  or  to  ascertain  what  effect  the  addition  of  a 
few  inches  to  the  rim  will  have  upon  the  stored  energy,  and 
upon  the  tension  in  the  rim  due  to  centrifugal  force. 
The  energy  stored  in  a  flywheel  is  expressed  thus — 

F  _  W  x  Y2 
64-33 

where  W  =  weight  of  rim  in  Ibs. 

V  =  velocity  in  feet  per  second  at  the  centre  of  the  mass 
of  the  rim,  or,  as  scientifically  expressed,  at  the 
radius  of  gyration 

Example. — What  energy  is  stored  in  the  flywheel  of  the  engine  shown 
by  Fig.  35?  We  must  first  find  the  weight  of  the  rim  in  pounds.  There 
are  two  ways  of  doing  this — one  is  to  take  from  a  table  of  areas  the  area 
of  a  circle  corresponding  with  the  outside  diameter  of  the  wheel,  then 
take  the  area  of  a  circle  corresponding  with  the  inside  of  the  rim,  and 
subtract  one  from  the  other  The  result  will  be  the  superficial  area  of  the 
rim.  In  the  case  in  point  the  diameter  of  the  wheel  is  3  i  inches  and  the 
area  1,017  inches ;  the  diameter  inside  the  rim  is  26  inches  and  the  area 
530  inches  ;  subtracting  one  from  the  other  we  have  487  inches  ;  the  rim 
is  5  inches  thick,  so  we  multiply  487  inches  by  5  inches,  and  the  result  is 
2,43  >  cubic  inches  of  iron.  We  read  in  Chapter  i.  that  1  cubic  inch  of 
cast  iron  weighs  '26  lb.,  so  that  if  we  multiply  2,435  by  '26  we  have  the 
weight  of  the  rim  in  pounds — viz.,  633  Ibs.  The  other  way  is  to  find  the 
circumference  of  a  circle  running  through  the  centre  of  the  rim, — 97 '4, 
and  to  multiply  this  by  the  sectional  area  of  the  rim — 25  inches — 
the  result  is  the  same — viz.,  2,435  cubic  inches.  Having  got  the  weight, 
we  must  find  out  what  V  is.  The  engine  runs  at  300  revolutions  per 
minute,  or  5  revolutions  per  second ;  if,  then,  we  take  the  circumference 
in  feet  of  the  wheel  at  the  centre  of  the  rim — viz. ,  at  the  point  indicated 

9 


130  MECHANICAL    ENGINEERING   FOR    BEGINNERS. 

by  a  cross  *  in  Fig.  35— and  multiply  it  by  the  number  of  revolutions  per 
second,  it  will  give  us  the  velocity  at  which  the  rotating  mass  is  moving. 
The  diameter  of  the  wheel  at  the  centre  of  the  rim  is  2  feet  7  inches,  or 
2-58  feet,  and  the  circumference  of  a  circle  2-58  diameter  is  8*1  feet;  if 
we  now  multiply  the  circumference  by  the  number  of  revolutions  per 
second,  we  get  the  answer  40*5  feet  per  second.  By  the  formula  we  are 
required  to  square  this  ;  the  square  of  40 '5  is  1,640.  The  calculation  now 

is  6^364.33640  =  16,137.     The  stored  energy  is,  therefore,  16,137  foot-lbs. 

If  the  student  will  work  out  the  energy  of  a  flywheel  having  a  rim  of 
the  same  section — viz. ,  5  inches — but  the  wheel  to  be  6  inches  larger  in 
diameter,  he  will  find  that  the  stored  energy  has  gone  up  greatly — viz.,  to 
27,000  foot-lbs. 

The  diameter  of  a  flywheel  is  governed  by  the  peripheral 
speed  at  which  it  is  safe  to  run.  It  is  usually  considered  that 
a  cast-iron  flywheel  should  not  run  at  a  peripheral  speed  greater 
than  6,000  feet  per  minute ;  at  this  speed  the  bursting  stress  in 
the  rim  due  to  centrifugal  force  is  only  about  972  Ibs.  per  square 
inch,  and  even  if  the  engine  is  called  upon  to  run  at  10  per  cent, 
above  its  normal  speed,  the  bursting  stress  will  only  be  about 
1,176  Ibs.  per  square  inch.  There  is,  of  course,  the  possibility 
of  the  engine  racing  to  be  taken  into  account,  but  even  should  it 
race  to  the  extent  of  50  per  cent.,  the  bursting  stress  will  be 
only  about  2,186  Ibs.  per  square  inch,  or  a  little  under  1  ton. 
There  is  no  doubt,  therefore,  that  a  peripheral  speed  of  6,000  feet 
per  minute,  or  100  feet  per  second,  is  a  very  safe  one  for  a  fly- 
wheel having  a  solid  rim.  If  the  flywheel  is  of  such  a  size  that 
the  rim  has  to  be  in  two  or  more  pieces,  then  the  safety,  or 

*  To  obtain  this  point  accurately  the  following  formula  should  be  used :  — 

+  B2 


where  A  =  outside  diameter. 
B  =  inside  diameter. 

This  formula,  which  may  perhaps  look  difficult  to  the  beginner,  is  really 
very  simple.  It  shows  that  we  square  the  outside  diameter  of  the  fly- 
wheel, then  squa:e  the  inside  diameter,  add  the  two  together,  divide  the 
product  by  two,  and  then  find  its  square  root  from  a  table  of  squares.  The 
calculation  is  as  follows  :  — 


,296  +  676 
~ ,  or 


A  table  of  squares  and  square  roots  will  show  that  the  square  root  of 
986  is  31 '4,  so  that  we  should  take  the  circumference  of  a  circle  3 1 '4 
diameter  instead  of  one  of  31  inches  diameter  obtained  by  measuring  to 
the  actual  centre  of  the  rim. 


THE    STEAM    ENGINE.  131 

otherwise,  depends  on  the  method  of  jointing,  and  a  speed  of 
4,000  feet  per  minute  is  probably  sufficiently  high.  With  fly- 
wheels of  very  large  size,  say,  20  feet  diameter  and  over,  it 
is  not  usual  to  run  at  a  peripheral  speed  much  higher  than 
3,500  feet  per  minute. 

Centrifugal  force  is  a  convenient  expression,  but  what  it  really 
represents  is  covered  by  Newton's  first  law  of  motion.  The  law 
reads  thus : — "  Every  body  continues  in  a  state  of  rest  or  of 
uniform  motion  in  a  straight  line  except  so  far  as  it  is  compelled 
by  force  to  change  that  state."  Now,  every  particle  in  the  rim 
of  a  flywheel  which  has  been  set  in  motion  tends  to  continue  in 
a  straight  line,  and  unless  force  is  employed  to  make  the  particle 
travel  in  a  circle,  the  particle  will  continue  to  move  in  a  straight 
line,  which  forms  a  tangent  to  the  circle.  The  force  which  has 
to  be  employed  to  make  the  particle  travel  in  a  curved  path  is 
the  equivalent  of  the  centrifugal  force  exerted. 

To  find  the  centrifugal  force  of  a  revolving  mass  the  following 
formula  is  used  : — 

W    *    V2 

c  =™ 


32-1   x  R' 

where  W  =  weight  in  pounds. 

V  =  velocity  in  feet  per  second. 
R  —  radius  in  feet. 

The  following  will,  however,  be  found  a  more  convenient 
formula : — 

C  =  W  x  N'2  x  R  x  1-226; 

where  W  =  weight  in  pounds. 

N  =  number  of  revolutions  per  second. 
R  =  radius  in  feet. 

Example. — What  is  the  centrifugal  force  of  a  piece  of  iron  1  inch 
square,  rotating  at  960  revs,  per  minute  at  a  radius  of  1  foot  ?  The  weight 
of  1  cubic  inch  of  cast  iron  is  '26  lb.,  and  960  revs,  per  minute  =  16  revs, 
per  second ;  the  square  of  16  is  256.  The  calculation  is,  therefore, 
•26  x  256  x  1  x  1-226.  The  answer  is  81 '6.  The  force  required  to  make 
a  piece  of  iron  1  inch  square  travel  in  a  circle  of  2  feet  at  the  speed 
mentioned,  instead  of  going  off  at  a  tangent,  is  81 '6  Ibs. 

Stress  set  up  in  the  Rim  of  a  Flywheel. — The  formula  given 
above  enables  one  to  find  the  centrifugal  force  of  any  portion  of 
the  rim  of  a  flywheel,  but  to  find  the  stress  set  up  in  the  rim 
and  tending  to  burst  it  as  the  result  of  centrifugal  force  of  all 
its  particles,  it  is  necessary  to  find  the  centrifugal  force  of 
1  cubic  inch  in  the  manner  given  above,  then  to  multiply  it 


132  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

by  the  diameter  of  the  wheel,  and  divide  the  result  by  2,  or 
expressed  as  a  formula  — 

C  x  P 


where  B  =  bursting  stress  per  square  inch  of  rim. 

C  =  centrifugal  force  of  1  cubic  inch  in  pounds. 
D  =  diameter  of  wheel  in  inches. 

The  centrifugal  force  is  multiplied  by  the  diameter,  and  not  by 
the  circumference  of  the  wheel,  for  the  same  reason  that  the 
stress  upon  a  cylinder  wall  under  internal  pressure  is  found  by 
multiplying  the  diameter  by  the  pressure.  The  reason  is  that 
the  pressure  on  the  circumference  is  not  all  equally  effective, 
and  the  total  effective  pressure  is  equivalent  to  the  pressure 
multiplied  by  the  diameter;  and,  as  in  a  cylinder  the  total 
pressure  is  borne  by  the  two  sides  of  a  cylinder,  so  in  a  flywheel 
the  total  bursting  pressure  is  borne  by  the  two  sides  of  the  rim  ; 
hence,  C  multiplied  by  D  is  divided  by  2. 

The  formula  gives  the  stress  on  each  square  inch  of  the  rim 
due  to  centrifugal  force,  and  it  is  immaterial  what  the  width 
may  be.  It  is  best  to  work  out  the  stress  in  a  ring  equal  to  the 
outside  diameter  of  the  wheel  and  of  1  square  inch  section. 
The  stress  on  the  inner  portion  of  the  rim,  assuming  the  latter 
is  greater  than  1  inch  square,  will,  of  course,  be  less  than  in  the 
outer  portion  ;  but  if  a  crack  develops  in  the  outer  portion,  the 
inner  portion  will  give  way  also. 

Example.  —  What  is  the  stress  in  the  rim  of  a  small  wheel,  2  feet 
diameter,  running  at  960  revs,  per  minute  ?  From  the  example  worked 
out  above  we  have  seen  that  the  centrifugal  force  of  1  inch  of  the  rim  is 
81*6  Ibs.;  we  therefore  multiply  this  by  the  diameter  —  viz.,  24  inches— 
and  divide  by  2.  The  answer  is  979  Ibs.  per  square  inch. 

A  very  simple  formula  for  finding  the  stress  per  square  inch 
in  the  rim  of  a  flywheel  is  given  by  Professor  Unwin  in  his 
book  on  machine  design.  It  is 

3-36  Y2 

32-1 
where  Y  =  velocity  in  feet  per  second. 

The  figures  3-36  represent  the  weight  of  1  foot  of  wrought 
iron  of  1  inch  area.  To  make  the  formula  applicable  to  cast- 
iron  flywheels,  3-12  should  be  used  instead  of  3-36. 

An  interesting  feature  about  the  centrifugal  force  acting  on 
flywheels  is  this  :  —  If  we  have  a  20-feet  wheel  running  at  95  -5 
revs.,  a  5-feet  wheel  running  at  382  revs.,  and  a  1-foot  wheel 


THE    STEAM    ENGINE.  133 

running  at  1,910  revs.,  the  peripheral  speed  is  the  same  in  each 
case,  but  1  cubic  inch  of  cast  iron  placed  on  the  rim  of  the  large 
wheel  exerts  a  centrifugal  force  of  8-1  Ibs.  only.  One  cubic  inch 
of  iron  placed  on  the  rim  of  the  5-feet  wheel  exerts  a  force  of 
32  Ibs.,  while  a  similar  piece  of  iron  placed  on  the  1-foot  wheel 
exerts  a  force  of  162  Ibs.  The  explanation  is  simple.  It  is 
this  : — The  path  pursued  by  the  piece  of  iron  on  the  rim  of  the 
large  wheel  approaches  more  nearly  to  a  straight  line  than  is 
the  case  of  the  piece  on  the  small  wheel,  and  a  smaller  force  is 
required  to  make  it  travel  in  the  necessary  curve.  In  the  small 
wheel  the  departure  from  the  straight  line  in  a  journey  of  1  foot 
is  much  greater,  hence  greater  force  is  required  to  hold  it  in. 

The  size  of  the  wheel  does  not,  however,  affect  the  bursting 
stress  per  square  inch  of  section  of  the  rim,  as,  although  in  the 
20-feet  wheel  the  centrifugal  force  of  1  inch  of  metal  is  only 
about  8'1  Ibs.,  there  is  the  equivalent  of  240  of  such  pieces  to 
be  reckoned  with,  while  in  the  1-foot  wheel,  although  the  force 
is  162  Ibs.,  there  are  only  12  such  pieces  to  be  allowed  for. 

The  above  formulae  for  finding  the  stress  in  the  rim  of  a 
flywheel  assume  that  the  tension  is  all  taken  by  the  rim,  the 
holding-in  power  of  the  arms  being  ignored.  In  flywheels  of  the 
disc  pattern,  as  used  on  high-speed  engines,  the  holding-in  power 
of  the  disc  is  very  considerable.  If  we  take  the  case  of  a  disc 
flywheel  having  a  rim  6  inches  wide  by  6  inches  deep,  the  disc 
1  inch  thick,  the  radius  to  the  centre  of  the  rim  1-5  feet,  and 
the  number  of  revolutions  660  per  minute,  we  shall  find  that, 
although  the  speed  is  6,200  feet  per  minute,  the  tension  on 
the  disc  (due  to  the  centrifugal  force  of  a  section  of  the  rim 
6  inches  x  6  inches  x  1  inch)  is  only  2,081  Ibs.  per  square 
inch,  so  bhat  even  if  the  rim  had  a  succession  of  saw  cuts  in  it 
6  inches  deep,  the  disc  would  hold  the  pieces  of  the  rim  in  place. 

The  author  knew  of  a  case  where  a  600  I.H.P.  high-speed 
engine  got  out  of  hand  while  being  run  for  the  purpose  of 
setting  the  governors ;  the  engine  finally  attained  such  a 
terrific  speed  that  it  was  completely  wrecked,  the  only  part 
remaining  undamaged  being  the  solid  disc  wheel. 

Stored  Energy  in  Flywheels. — It  may  be  asked  what  amount 
of  stored  energy  should  a  flywheel  possess  1  The  answer 
depends  upon  the  type  of  engine  to  which  the  wheel  is  to 
be  fitted,  the  work  the  engine  has  to  do,  and  the  regularity 
of  turning  that  is  required.  For  instance,  a  double-acting  engine 
having  three  cranks  placed  at  120°  apart  will  not  require  such  a 
heavy  flywheel  as  an  engine  with  two  cranks  at  180°  apart.  An 
engine  which  has  to  drive  a  large  circular  saw  and  a  dynamo 
will  require  a  much  heavier  wheel  than  one  driving  several 


134  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

small  lathes  and  a  dynamo,  for,  in  the  case  of  the  saw,  the  whole 
load  comes  off  and  on  suddenly.  Again,  an  engine  which  is 
required  to  drive  a  three-phase  alternator  in  parallel  with 
others  will  need  a  heavier  flywheel  than  a  similar  engine  to 
be  used  for  driving  the  tools  in  an  engineer's  shop.  To  go 
deeply  into  the  question  of  flywheel  weights  and  fluctuations 
of  speed,  taking  into  consideration  the  varying  pressures  on  the 
crank-pin  due  to  steam  pressure,  inertia  of  moving  parts,  &c.,  is 
beyond  the  scope  of  this  book,  and  unless  the  student  is  engaged 
with  a  firm  of  engine  builders,  he  need  hardly  concern  himself 
with  such  calculations.  Even  in  engine  builders'  works  the 
amount  of  stored  energy  of  the  flywheel  is  usually  settled 
upon  as  the  result  of  previous  experience  rather  than  as  the 
result  of  calculation.  For  instance,  it  was  found  that  with  a 
certain  type  of  single-acting  three-crank  engine,  a  flywheel 
having  stored  energy  of  3,000  to  4,000  ft.-lbs.  per  I.H.P.  was 
sufficient  to  meet  the  most  onerous  conditions,  and  a  flywheel 
having  this  amount  of  energy  was  accordingly  provided  when 
the  price  obtained  admitted  of  such  a  heavy  flywheel. 

In  the  huge  flywheel,  having  a  wrought-iron  rim,  fitted  to 
the  earlier  5,000  H.P.  turbines  at  the  Niagara  Falls,  the  stored 
energy  works  out  at  about  22,000  Ibs.  per  H.P.,  but  the  condi- 
tions necessitating  a  flywheel  having  such  an  enormous  amount 
of  stored  energy  are  quite  unusual. 

Governing  Steam  Engines. — The  speed  of  a  steam  engine 
is  controlled  by  the  governor  and  flywheel ;  the  former  controls 
the  normal  speed  (sometimes  called  steady  speed),  while  the  latter 
prevents  any  sudden  variations  of  speed,  and  gives  the  governor 
time  to  act.  The  governor  may  regulate  the  speed  either  by 
throttling  the  steam  before  it  enters  the  valve  chest,  or  by 
making  the  cut-off  take  place  earlier  or  later  during  the  stroke. 

In  the  ordinary  throttle  governor  the  position  of  the  balls 
determines  the  amount  of  opening  of  the  throttle  valve,  and 
centrifugal  force  determines  the  position  of  the  balls.  The 
valve  is  set  so  that  when  the  engine  is  running  at  full  speed  the 
passage  of  steam  is  unobstructed,  or  obstructed  only  so  far  as  to 
give  a  little  higher  pressure  on  the  boiler  side.  If  the  speed  of 
the  engine  increases  beyond  the  full  speed  for  which  it  was 
designed,  the  balls  open  outwards  and  the  throttle  valve  partially 
closes ;  if  the  speed  of  the  engine  falls,  the  balls  close  either 
by  their  own  weight,  or  assisted  by  springs,  and  the  throttle 
valve. opens.  When  the  engine  is  at  rest  the  throttle  valve  is 
wide  open. 

In  a  governor  which  controls  the  speed  of  the  engine  by 
varying  the  expansion,  as  in  a  Corliss  engine,  the  motion  of  the 


THE    STEAM    ENGINE.  135 

balls  causes  the  catch  plates  to  release  the  steam  admission  valve 
at  an  earlier  or  later  portion  of  the  stroke. 

It  should  be  clearly  understood  that  a  governor,  however 
sensitive,  cannot  govern  an  engine  to  one  absolutely  uniform 
speed,  as  it  is  only  by  a  variation  of  the  speed  that  the  centri- 
fugal governor  comes  into  play.  A  good  governor  should, 
however,  be  able  to  control  the  steady  speed  of  the  engine  to 
within  3  per  cent,  under  all  changes  of  load,  so  that  if  the 
governor  is  set  to  give  a  speed  of  100  revs,  per  minute  under 
full  load  the  engine  should  never  run  faster  than  103  revs,  even 
without  any  load.  If  the  whole  load  is  thrown  off  suddenly, 
the  speed  may  rise  momentarily  beyond  the  103  revs.,  but  the 
governor  will  bring  it  back  in  a  few  seconds.  The  extent  of  the 
momentary  variation  of  speed  depends  more  upon  the  flywheel 
than  upon  the  governor;  therefore  the  flywheel  should  be 
sufficiently  heavy  to  keep  the  momentary  variation  of  speed 
down  to  5  per  cent,  until  the  governor  is  able  to  produce  its 
effect.  When  a  complaint  is  made  that  an  engine  will  not 
govern  under  sudden  changes  of  load  the  fault  may  be  looked 
for  rather  in  the  flywheel  than  in  the  governor. 

A  typical  specification  as  to  governing  is  the  following : — 
"  The  governor  to  be  of  an  approved  type  capable  of  easy 
adjustment  by  hand  while  the  engine  is  running.  The 
governor  to  control  the  speed  of  the  engine  within  3  per 
cent,  between  full  load  and  no  load,  and  with  a  temporary 
variation  of  not  more  than  5  per  cent,  under  any  variation 
of  load." 

If  an  attempt  is  made  to  govern  an  engine  much  closer  than 
is  indicated  by  the  above  limits,  or  to  make  the  governor 
"isochronous,"  there  is  a  tendency  for  the  governor  to  hunt. 
What  is  called  hunting  is  this  ;  when  the  governor  is  fitted  with 
too  sensitive  springs  and  an  increase  of  speed  takes  place,  the 
throttle  valve  closes  suddenly  and  thrott'es  the  steam  to  such  an 
extent  that  the  speed  of  the  engine  falls  below  the  normal ;  the 
throttle  valve  then  opens  suddenly  and  allows  rather  too  much 
steam  to  pass,  when  the  speed  rises  above  the  normal ;  the 
governor  again  closes  suddenly,  and  so  the  hunting  action  goes 
on.  This  hunting  may  give  rise  to  very  irregular  running,  and 
it  is  preferable  to  have  a  governor  that  will  control  the  speed  of 
the  engine  within  reasonable  limits,  and  be  "stable,"  than  to 
have  one  fitted  with  such  light  springs  that  it  will  hunt  on 
the  least  provocation. 

The  balls  of  a  governor  should  be  fairly  heavy,  and  con- 
trolled by  strong  springs  if  the  spindle  is  horizontal,  or  by  a 
weight  if  vertical.  If  the  balls  are  light  and  the  springs  weak 


136  MECHANICAL    ENGINEERING   FOR    BEGINNERS. 

any  friction  of  the  governor  mechanism  may  seriously  interfere 
with  the  governing. 

It  is  more  difficult  to  govern  within  close  limits  a  compound 
or  triple-expansion  engine  than  a  simple  engine  unless  the  cut-off 
can  be  varied  in  each  cylinder,  as  there  is  a  certain  amount  of 
steam  in  the  receivers  which  must  be  got  rid  of  before  the  full 
effect  of  throttling  or  cutting  off  the  admission  in  the  H.P. 
cylinder  is  felt. 

Some  engines  are  fitted  with  an  emergency  governor.  This 
governor  does  not  come  into  action  unless  the  main  governor 
fails  to  hold  the  engine,  and  the  speed  assumes  dangerous  pro- 
portions. The  governor  of  an  engine  should  not  be  driven  by  a 
belt,  as  this  may  slip  or  break.  In  high-speed  engines  it  is  a 
common  practice  to  fit  the  governor  to  one  end  of  the  crank 
shaft,  and  thus  do  away  with  any  gearing ;  in  cases  where  this 
is  not  practicable  the  governor  should  be  driven  by  gearing. 

Relative  Economy  of  Governing  by  Throttling  or  by 
Varying  the  Expansion. — It  is  generally  believed  that  it  is 
more  economical  to  govern  an  engine  by  altering  the  point  of 
cut-off  than  to  govern  it  by  throttling  down  the  steam  at  light 
loads.  It  is  contended  that  if  the  boiler  pressure  is,  let  us  say, 
160  Ibs.,  it  is  better  to  work  with  this  pressure  at  all  loads 
rather  than  throttle  down  the  steam  to  60  or  70  Ibs.  at  very 
light  loads.  This  is,  in  the  main,  correct,  and  with  low  admis- 
sion pressure  it  is  undoubtedly  more  economical  to  vary  the 
expansion  whatever  the  load  may  be  than  to  further  throttle 
down  the  steam ;  but  when  high  initial  pressures  have  to  be 
dealt  with  the  case  is  rather  different,  for  if  the  expansion  is 
carried  out  to  too  great  an  extent  the  range  of  temperature  in 
the  H.P.  cylinder  is  very  great,  and  the  initial  condensation 
is  excessive. 

If  the  load  on  the  engine  is  so  light  that  the  steam  is  expanded 
until  the  mean  pressure  in  the  H.P.  cylinder  is  about  one-tenth 
of  the  admission  pressure,  then  it  is  more  economical  to  throttle 
down  the  steam  and  to  expand  less. 

For  any  variations  of  load  less  than  the  above,  the  variable 
expansion  type  of  governor  is  more  economical  than  a  governor 
of  the  throttling  type. 

In  cases  where  an  overload  may  come  upon  the  engine,  the 
variable  expansion  type  of  governor  is  far  preferable  to  the 
throttling  type.  For  instance,  an  engine  is  fitted  with  a  throttle 
governor,  and  the  full  load  is  obtained  with,  say,  160  Ibs.  pres- 
sure and  -5  cut-off;  if  a  sudden  overload  should  come  upon  the 
engine  it  cannot  respond,  because  the  governor  obviously  cannot 
raise  the  steam  pressure  above  that  of  the  boiler,  and  the  point 


THE    STEAM    ENGINE. 


137 


of  cut-off  is  fixed  •  whereas  if  the  engine  is  fitted  with  variable 
expansion  gear,  as  soon  as  the  speed  of  the  engine  begins  to  fall 
the  governor  makes  the  point  of  cut-off  later,  and  so  increased 
power  is  obtained. 

Proportions  of,  and  Stresses  in,  Various  Parts  of  a 
Steam  Engine. — The  following  data,  which  are  based  upon 
present-day  practice,  may  be  useful : — 

Ratio  of  Cylinders. — The  ratios  which  the  intermediate  and 
L.P.  cylinders  bear  to  the  H.P.  cylinder  are  usually  as  follows  : — 


Compound  Non- 
condensing  Engines. 

Compound 
Condensing  Engines. 

Triple  Condensing  Engines. 

H.P. 

L.P. 

H.P. 

L.P. 

H.P. 

I.P. 

L.P. 

2-5 

3-0 

2-25 

6-0 

1 

to 

1 

to 

1 

to 

to 

30 

3-5 

2-5 

6'5 

Area  of  Ports. — The  area  of  the  steam  ports  should  be  such 
that  the  flow  of  steam  through  them  does  not  exceed  7,000  feet 
per  minute.  In  the  case  of  a  locomotive  running  at  60  miles 
per  hour,  the  speed  of  the  steam  through  the  ports  is  sometimes 
as  high  as  11,000  feet  per  minute,  but  at  this  speed  the  steam  is 
seriously  throttled.  The  area  of  the  exhaust  ports  of  Corliss 
engines  is  usually  about  1J  times  that  of  the  steam  ports.  In 
the  case  of  slide-valve  engines  the  exhaust  port  is  made  from 
1J  to  3  times  the  width  of  the  steam  ports.  The  exhaust  port 
of  the  L.P.  cylinder  which  communicates  with  the  condenser 
should  be  made  as  large  as  the  general  design  of  the  cylinder 
will  admit  of. 

Having  decided  upon  the  speed  at  which  the  steam  is  per- 
mitted to  pass  through  the  ports,  the  area  is  found  thus — 


Area  of  port  in  inches  = 


where  P  =  piston  area  in  inches. 

S  =  piston  speed  in  feet  per  minute. 

V  =  velocity  in  feet  per  minute  at  which  steam  is  per- 
mitted to  pass  through  port. 

Thickness  of  Cylinder  Walls. — The  thickness  of  a  cylinder 
wall  is  such  that  the  stress  upon  it  is  about  1,500  Ibs.  per 
square  inch,  with  a  minimum  thickness  of  J-  inch.  The  thick- 


138  MECHANICAL    ENGINEERING   FOR    BEGINNERS. 

ness  (without   reference   to   the   minimum)   can,   therefore,   be 
found  as  follows  : — 

5^J?_T. 

3,000    " 

where  D  =  diameter  of  cylinder  in  inches. 
P  =  highest  admission  pressure. 
T  =  thickness  of  cylinder  in  inches. 

The  thickness  of  the  H.P.  and  L.P.  walls  is  usually  the  same. 

Piston-rod. — This  must  be  of  sufficient  size  to  withstand  the 
alternate  tension  and  compression  due  to  the  pressure  on  the 
piston  (and  to  its  inertia) ;  it  must  also  be  sufficiently  stiff  to 
resist  any  tendency  to  bend  or  buckle  under  its  load.  The  stress 
usually  allowed  is  about  3,000  Ibs.  upon  each  square  inch  of  the 
full  diameter  of  the  rod ;  the  stress  at  the  smallest  part  of  the 
rod — viz.,  at  the  bottom  of  the  thread  cut  for  the  nut  used  to 
hold  the  piston,  the  rod  having  previously  been  tapered  down 
for  the  piston — may  be  as  much  as  5,500  or  6,000  Ibs.  per  square 
inch.  In  locomotive  practice  the  rod  is  usually  enlarged  where 
it  fits  into  the  piston,  and  before  the  taper  begins ;  even  then 
stresses  up  to  7,500  Ibs.  per  square  inch  are  often  met  with. 
The  piston-rods  of  the  H.P.,  I. P.,  and  L.P.  pistons  are  usually 
of  the  same  diameter,  and  it  will  be  found  that  this  is  usually 
from  -fa  to  y1^  the  diameter  of  the  L.P.  piston.  In  large  marine 
engines,  or  in  engines  where  all  the  parts  are  light,  the  rod  is 
often  only  y1^  the  diameter  of  the  L.P.  piston,  but  in  the  case 
of  Corliss  engines  and  of  compound  locomotives  the  diameter 
is  usually  about  J  that  of  the  L.P.  piston. 

Connecting-rods. — The  usual  practice  as  regards  length  of 
connecting-rod  is  as  follows : — 

Marine  engine,  3  -5    to  5-0  times  length  (or  radius)  of  crank. 
Stationary           I      ; 

land  engines.  J 

Locomotives,  5*5    to7'0*        ,,  „  „ 

The  sectional  area  of  the  connecting-rod  should  be  equal  in  its 
smallest  part  to  that  of  the  piston-rod,  and  the  section  should 
increase  gradually  towards  the  crank-pin  end  until  it  is  1J  to  1 J 
times  the  area  of  the  piston-rod. 

Eccentric-rods  and  Valve-rods. — The  size  of  these  depends 
largely  upon  the  character  of  the  valve  gear,  and  no  general 
rules  can  be  given. 

*  In  the  case  of  a  four-cylinder,  ten-coupled  locomotive  recently  con- 
structed by  the  Austro- Hungarian  Railway,  the  connecting-rod  was  9 '4 
times  the  length  of  the  crank. 


THE    STEAM    ENGINE,  139 

Pressure  on  Journals,  Crank-pins,  and  Crosshead-pins.  —  The 
pressure  which  may  safely  be  allowed  depends  to  a  great 
extent  upon  the  speed  of  the  rubbing  surfaces ;  thus  a  much 
greater  pressure  is  usually  allowed  upon  the  crosshead-pin  than 
upon  the  main  bearings.  The  pressures  usually  allowed  are  as 
follows : — 

Main  bearings,      .        300  to     500  Ibs,  per  square  inch. 
Crank-pins,  .         .        600  to     900    „  „ 

Crosshead-pins,     .     1,200  to  1,500*  „  „ 

The  above  pressures  are  reckoned  on  the  projected  area  of  the 
bearing—  i.e.,  the  diameter  multiplied  by  its  length,  and  not  half 
the  circumference  multiplied  by  the  length. 

Diameter  of  Crank-shafts.  —  Crank-shafts  of  compound  and 
triple-expansion  engines  of  the  marine  type  are  usually  made 
approximately  one-fifth  the  diameter  of  the  L.P.  cylinder. 
Where  there  are  two  L.P.  cylinders  to  one  H.P.  or  intermediate 
cylinder,  then  the  crank-shaft  will  be  found  to  be  approximately 
one-fifth  the  diameter  of  a  cylinder  having  an  area  equivalent 
to  the  two  L.P.  cylinders.  In  the  case  of  high-speed  engines, 
where  the  reversal  of  stress  in  the  shaft  is  very  frequent,  the 
crank-shaft  is  usually  made  about  one-quarter  the  diameter  of 
the  L.P.  cylinder.  This  rule  does  not  apply  to  the  crank-shafts 
of  gas  engines  or  to  engines  which  have  a  heavy  flywheel  or  rope 
pulley  placed  between  the  cranks,  as  is  frequently  the  case  with 
horizontal  land  engines. 

Pressure  upon  Slide  Bars. — The  pressure  upon  the  slide  bars 
is  found  by  multiplying  the  total  pressure  on  the  piston,  by 
the  length  of  crank,  and  dividing  the  result  by  the  length  of 
connecting-rod.  There  is  usually  no  difficulty  in  providing  slide 
bars  and  slipper  having  ample  surface,  and  the  pressures  upon 
them  are  consequently  light ;  they  vary  from  50  Ibs.  per  square 
inch  of  surface  in  stationary  engines  to  about  125  Ibs.  per  square 
inch  in  the  case  of  marine  engines  and  locomotives.  It  may  be 
stated  here  that  friction  theoretically  is  independent  of  the 
extent  of  rubbing  surface,  and  depends  only  upon  the  pressure 
between  the  surfaces  in  contact.  The  coefficient  of  friction  of 
dry  steel  to  steel  is  about  '18,  so  that  if  we  have  a  piece  of  steel 
weighing  100  Ibs.  pressing  on  another  piece,  it  will  require  about 
18  Ibs.  to  move  it  about.  If,  however,  we  can  keep  a  film  of  oil 
between  the  surfaces  the  friction  is  considerably  reduced.  The 
best  way  to  ensure  a  film  of  oil  remaining  between  the  surfaces 
is  to  provide  ample  area,  and  consequently  reduced  pressure. 

*  Exceeded  in  locomotives. 


HO 


MECHANICAL  .ENGINEERING   FOR    BEGINNERS. 


Some  experiments  have  been  carried  out  at  Cooper's  Hill 
Engineering  College*  in  order  to  ascertain  the  coefficient  of 
friction  between  wrought  iron  and  steel  journals  and  bearings 
of  different  alloys,  the  journal  and  its  bearing  being  immersed 
in  a  bath  of  oil.  The  following  are  approximately  the  results 
obtained  with  a  steel  journal  running  in  a  phosphor-bronze 
bearing  : — 


Coefficient  ( 

)f  Friction. 

Pressure  on  Bearing  per 
Square  Inch. 

When  Peripheral  Speed 
of  Journal  =  400  Feet 

When  Peripheral  Speed 
of  Journal  =  100  Feet 

per  Minute. 

per  Minute. 

600  Ibs. 

•0032 

•0029 

500 

•0035 

•0029 

400 

•0040 

•0030 

300 

•0045 

•0033 

200 

•0060 

•0040 

100 

•0095 

•0054 

It  is  probably  impossible  to  ensure  such  a  good  film  of  oil 
remaining  between  sliding  surfaces  as  between  a  shaft  and 
bearing  running  in  oil,  as  the  shaft  draws  in  oil  in  the  same 
way  that  a  pair  of  rolls  draws  in  a  sheet  of  metal.  It  is, 
however,  advisable  to  provide  large  rubbing  surfaces,  so  that 
the  oil  is  not  so  easily  squeezed  out,  and  in  this  sense  the 
statement  that  friction  is  independent  of  surface  has  to  be 
qualified. 

Piston  Speeds. — The  piston  speeds  usually  found  in  actual 
practice  are  approximately  as  follows  : — 


Corliss  engines, 
Marine  engines, 
Locomotives,  . 


.     500  feet  per  minute. 

.     500  to  1,200  feet  per  minute. 

1,120  feet  per  minute  when  running 
at  60  miles  per  hour. 

Engine  Packings. — In  the  early  days  of  the  steam  engine, 
when  steam  pressures  were  low,  it  was  customary  to  pack  the 
glands  of  the  piston-  and  valve-rods  with  hemp  soaked  in  tallow, 
but  with  the  advent  of  high  pressures  it  was  found  that  hemp 
charred,  owing  to  the  heat.  When  this  occurred,  the  engine 
driver,  in  order  to  keep  the  gland  tight,  screwed  down  his  gland 
tighter  and  tighter;  the  result  was  frequently  a  scored  rod. 
Asbestos  fibre  packing  was  introduced  about  the  year  1870,  and 

*  See  Engineering,  vol.  Ixxxii, ,  p.  595. 


THE    STEAM    ENGINE. 


141 


was  found  far  superior  to  hemp  for  high-pressure  steam,  and  it 
is  very  largely  used  at  the  present  time.  The  method  of  packing 
a  gland  by  squeezing  any  form  of  fibrous  material  up  against  the 
rod  and  walls  of  the  gland  is,  however,  open  to  objection.  In 
the  first  place,  if  the  driver  screws  down  his  gland  nuts  too 
hard,  an  excessive  amount  of  friction  on  the  rod  is  caused;  it  is 
possible  to  pull  up  an  engine  of  about  50  H.P.  by  tightening 
the  glands  excessively.  Secondly,  the  pressure  on  the  rod  is  the 
same  on  both  the  steam  and  exhaust  strokes.  Thirdly,  any  slight 
lateral  movement  of  the  rod  tends  to  cause  the  packing  to  leak. 


Fig.  49.— Metallic  packing. 

To  overcome  these  defects  various  forms  of  metallic  packing, 
consisting  chiefly  of  soft  white  anti-friction  metal  rings  or  blocks, 
have  been  designed  and  patented.  The  best  form  of  packing  of 
which  the  author  has  any  knowledge  is  that  shown  by  Fig.  49. 
This  packing  was  designed  by  a  Scotch  engineer,  Mr.  Monroe, 
patented  in  the  United  States,  and  is  made  and  sold  in  this 
country  under  the  slightly  misleading  name  of  the  United 
States  Metallic  Packing.  Fig.  49  shows  the  packing  as  supplied 


142  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

for  very  high  pressures  ;  for  low-pressure  cylinders  the  left-hand 
part  of  the  packing  only  is  required.  The  packing  consists  of 
eight  white  metal  blocks,  arranged  so  that  they  break  joint; 
each  block  is  pressed  on  to  the  rod  by  a  light  spring,  so  that  the 
pressure  on  the  rod  is  about  1J  Ibs.  per  square  inch  only;  on 
the  steam  stroke,  however,  the  pressure  of  the  steam  is  added  to 
that  of  the  springs,  and  a  tight  gland  is  the  result.  On  the 
exhaust  stroke  the  pressure  of  the  steam  is  removed,  the 
pressure  of  the  springs  being  sufficient  to  ensure  the  gland 
remaining  tight  under  low  pressure,  or  under  atmospheric 
pressure  when  the  engine  is  condensing.  The  white  metal 
rings  shown  in  the  right-hand  portion  of  the  packing  are 
for  the  purpose  of  reducing  the  pressure  of  the  steam,  when  it 
is  very  high,  before  it  reaches  the  packing  blocks  already 
referred  to.  As  the  reader  will  see  from  the  illustration,  the 
packing  admits  of  lateral  play  of  the  rod  without  affecting  the 
steam  tightness;  also,  that  it  is  impossible  for  the  driver  to  put 
any  undue  pressure  on  the  rod,  as  there  are  no  gland  nuts  to 
tighten. 

The  form  of  packing  described  is  very  largely  used  in  marine 
as  well  as  in  land  work ;  the  only  objection  to  it  is  that  the  first 
cost  is  greater  than  that  of  a  gland  arranged  to  receive  asbestos 
packing,  and  that  occasionally  the  springs  break.  Against 
these  objections  it  is  claimed  that,  by  reducing  the  friction  on 
the  rod,  the  mechanical  efficiency  of  the  engine  is  increased  by 
2  per  cent,  or  more,  and  that  the  cost  of  replacing  a  spring  is 
trifling. 

Piston  Rings. — In  the  very  early  days  of  the  steam  engine 
the  piston  was  packed  by  means  of  rope  or  "junk,"  fitted  into  a 
recess,  and  held  in  position  by  a  junk  ring.  The  first  to  depart 
from  this  plan  was  Mr.  Ramsbottom,  who  used  narrow  piston 
rings  as  shown  by  Fig.  52.  These  rings  were  turned  slightly 
larger  than  the  cylinder,  were  cut  across,  and  sprung  inwards ; 
the  spring  in  the  material  was  sufficient  to  cause  the  rings  to 
press  tightly  against  the  walls  of  the  cylinder.  Rin^s  of  this 
description  are  still  largely  used  in  locomotives  and  in  fast- 
running  petrol  engines.  There  is  nothing  in  them  to  go  wrong, 
and  they  do  not  require  a  junk  ring. 

The  rings  are  generally  cut  through  at  an  angle,  as  shown  by 
Fig.  50,  so  that  there  is  not  a  direct  path  for  the  steam  to  blow 
through.  When  the  rings  are  put  into  the  piston  the  openings 
.are  spaced  equally  round  the  piston,  but  it  is  found  that  the 
rings  have  a  tendency  to  work  round  into  the  position  shown  by 
Fig.  50.  It  is  said  that  this  tendency  can  be  frustrated  by 
•cutting  through  the  central  spring  at  an  angle  inclined  in  the 


THE    STEAM    ENGINE. 


143 


opposite  direction  to  the  angle  of  the  others,  as  shown  by  Fig. 
51.     The  objection  to  the  Ramsbottom  form  of  ring  is  that  it 


Fig.  50. 


Fig.  53. 


Fig.  56. 


Fig.  51. 


Fig.  54. 


Piston  rings. 


Fig.  52. 


Fig.  55. 


Fig.  57. 


cannot  be  replaced  without  drawing  the  piston,  unless  the  latter 
is  specially  constructed,  as  shown  by  Fig.  38,  and  that,  being 


144  MECHANICAL   ENGINEERING    FOR    BEGINNERS. 

originally  made  of  slightly  larger  diameter  than  the  cylinder,  it 
does  not  press  equally  all  round  the  latter  when  sprung  in. 

A  simple  form  of  piston  packing,  and  one  which  has  proved 
efficient  in  sizes  up  to  4.:  inches  diameter,  is  that  used  in  the 
Willans  engine ;  a  sectional  view  of  the  packing  is  shown  by 
Fig.  55.  The  packing  consists  of  two  rings,  A,  A,  turned  to  the 
exact  diameter  of  the  cylinder,  so  that  they  bear  equally  all  the 
way  round,  and  one  cast-iron  inner  spring,  B ;  this  spring  is 
turned  slightly  larger  than  the  bore  of  the  cylinder,  and  is 
thicker  at  one  portion  than  at  another,  as  shown  by  Fig.  53 ;  it 
is  then  cut,  sprung  inwards  and  placed  inside  the  two  thinner 
and  concentric  rings,  A.  The  tendency  of  the  spring  B  to 
resume  its  original  form  makes  the  rings  A  press  evenly  against 
the  cylinder  walls.  The  piston  rings  and  spring  require  to  be 
an  extremely  good  fit  between  the  junk  ring  and  piston  flange  C, 
otherwise  there  is  play,  and  the  springs  hammer  themselves  into 
the  junk  ring  and  piston  flange.  The  effect  was  tried  of  making 
the  junk  ring  of  thin  steel  plate,  the  idea  being  that  the  steam 
would  spring  the  steel  plate  on  to  the  rings  and  spring,  and  thus 
prevent  any  play,  but  the  experiment  was  not  a  success. 

In  the  Mudd  form  of  packing,  shown  by  Figs.  56  and  57,  any 
play  of  the  rings  between  the  junk  ring  and  piston  flange  is 
prevented  by  means  of  a  number  of  spiral  springs  as  shown. 
The  rings  are  kept  up  against  the  cylinder  walls  by  means  of 
springs  placed  between  the  ends  of  the  former,  as  shown  at  S,  S 
in  the  small  scale  plan  of  the  ring,  Fig.  6.  This  form  of  packing 
is  used  largely  in  marine  work,  and  has  proved  satisfactory. 
There  are  a  very  large  number  of  patent  piston  rings  and  springs 
besides  those  mentioned,  but  the  examples  chosen  will  seive,  as 
well  as  any,  to  make  the  principles  clear. 

Efficiency  of  Steam  Engines. — The  term  efficiency  is  often 
used  in  widely  different  senses.  For  instance,  when  a  man  says 
that  his  engine,  although  rather  old-fashioned,  is  still  very 
efficient,  he  probably  means  that  the  engine  does  not  break 
down  or  cause  trouble,  and  that  he,  the  owner,  is  ignorant  of, 
and  indifferent  to,  the  consumption  of  steam.  Another  man,  in 
stating  that  his  new  engine  is  extremely  efficient,  probably  refers 
to  the  consumption  of  steam  per  indicated  horse-power ;  while  a 
third,  in  endeavouring  to  sell  a  somewhat  uneconomical  engine, 
may  lay  stress  upon  its  high  mechanical  efficiency.  The  mechani- 
cal efficiency  of  an  engine,  as  already  explained,  depends  solely 
upon  the  amount  of  internal  friction. 

The  real  efficiency,  from  the  purchaser's  point  of  view,  is  the 
consumption  of  steam  at  a  given  pressure  (and  temperature)  per 
brake  or  effective  horse-power. 


THE    STEAM    ENGINE.  145 

The  true  thermal  efficiency  of  an  engine  is  the  ratio  the  number 
of  thermal  units  represented  by  the  actual  horse-power  developed, 
bears  to  the  number  of  thermal  units  put  into  the  steam  con- 
sumed by  the  engine.  The  efficiency  of  an  engine  reckoned  in 
this  way  is  so  low  (about  •  1 57  in  the  case  of  a  compound  con- 
densing engine,  using  15  Ibs.  of  steam  per  I.H.P.  per  hour, 
working  with  150  Ibs.  boiler  pressure)  that  it  is  seldom  used  in 
commerce,  and  is  chiefly  useful  in  comparing  the  performance  of 
a  steam  engine  with  that  of,  say  a  gas  engine. 

Then,  again,  the  thermal  efficiency  of  a  steam  engine  may  be 
compared  with  that  of  a  perfect  steam  engine  working  within 
the  given  limits  of  admission  and  exhaust  temperatures.  Such 
a  standard  has  been  recommended  by  the  Institution  of  Civil 
Engineers  (see  vol.  cxxxiv.,  p.  294  of  these  Proceedings}*  The 
standard  chosen  is  that  laid  down  by  Clausius  and  Rankine,  and 
not  that  of  the  Carnot  heat  cycle. 

Before  concluding  the  chapter  on  steam  engines,  a  few  words 
must  be  said  on  the  subject  of  the  Zeuner  diagram.  This 
diagram  enables  the  designer  to  see  at  what  part  of  the  stroke 
the  cut-off  takes  place  with  a  given  amount  of  lap  and  lead ; 
also  the  points  at  which  the  port  opens  to  exhaust,  and  where 
compression  begins. 

Perhaps  a  little  incident  which  actually  occurred  may  show 
the  beginner  how  the  ability  to  construct  such  a  diagram  helped 
at  least  one  young  draughtsman  a  step  forward.  This  young 
fellow,  who  wished  to  gain  further  experience,  accepted  a  berth 
as  draughtsman  with  a  small  firm  of  mechanical  engineers  on  the 
coast.  The  firm  had  under  construction  a  small  marine  engine 
of  a  size  not  previously  made,  and  when  it  came  to  drawing 
out  the  slide-valve,  the  principal,  who  was  an  extremely  good 
practical  engineer,  but  who  had  not  much  theoretical  knowledge, 
said  to  the  draughtsman — "  You  had  better  make  the  lap  three- 
quarters  of  an  inch ;  I  think  that  will  be  about  right."  The 
reply  was — "  Very  well,  sir,  if  you  like  I  will  set  out  a  Zeuner 
diagram,  so  that  we  may  see  what  the  effect  will  be."  The 
principal  replied — "Oh,  I  think  three-quarters  of  an  inch  will 
be  near  enough."  However,  after  office  hours,  the  draughtsman, 
for  his  own  satisfaction,  set  out  the  Zeuner  diagram,  and  the 
following  morning  showed  it  to  his  principal.  The  principal, 
whose  guess  as  to  the  right  amount  of  lap  had  been  a  good  one, 

*  These  volumes  can  be  seen  by  anyone  in  London  free  of  charge  at  the 
library  of  the  Patent  Office  in  Southampton  Buildings,  Chancery  Lane. 
This  library  contains  the  past  volumes  of  the  Engineer  and  Engineering, 
and  other  technical  papers  ;  also  a  large  collection  of  valuable  text-books 
dealing  with  various  subjects. 

10 


146 


MECHANICAL    ENGINEERING   FOR    BEGINNERS. 


seemed  very  pleased  to  see  set  out  so  clearly  the  exact  points 
where  cut-off  took  place  and  where  compression  began,  and  at 
the  end  of  the  week  the  draughtsman  was  gratified  to  find  that 

Fig.  58. 


E  xhaust 
Closes 


Fig.  59. — Zeuner  valve  diagram. 


his  salary  had  been  increased.  The  principal,  when  thanked, 
merely  said — "You  are  worth  more  than  you  are  getting." 
Had  the  principal's  guess  turned  out  a  bad  one,  it  is  just  possible 


THE    STEAM    ENGINE.  147 

that  he  might  not  have  been  so  pleased  with  the  Zeuner  diagram, 
but  it  would  have  been  accurate  just  the  same. 

The  diagram  (Fig.  58)  is  constructed  as  follows : — Draw  AB 
parallel  to  the  line  of  the  stroke.  With  a  radius  =  J  the  valve 
travel  draw  from  the  centre  E  the  circle  BCAK.  Mark  off 
EF  =  the  lap,  and  FG  =  the  lead.  Draw  the  perpendicular 
GH.  Join  EH.  Then  V  =  angular  advance.  On  HK  draw 
the  valve  circles  as  shown.  From  E  draw  lap  circles  with  radii 
EF  =  outside  lap  and  ER  =  inside  lap  ;  then — 

EB  =  position  of  crank  at  beginning  of  stroke. 
EM  =          „  „  at  cut-off. 

EN  —          „  ,,  when  exhaust  opens. 

EO  =          „  „  „          „        closes. 

EP  =          ,,  ,,  ,,     steam  port  opens. 

Having  found  the  position  of  the  crank  at  the  points  of 
admission,  cut-off,  compression,  &c.,  one  has  merely  to  draw  the 
connecting-rod  and  cylinder,  as  shown  by  Fig.  59,  to  obtain 
the  corresponding  position  of  the  piston.  In  the  illustration  the 
connecting-rod  has  been  assumed  to  be  five  times  the  length  of 
the  crank ;  EM  is  assumed  to  be  the  crank ;  therefore  the 
connecting-rod  is  drawn  five  times  the  length  of  EM.  One  end 
of  the  cylinder  will  then  be  a  connecting-rod's  length  from  A 
and  the  other  end  a  rod's  length  from  B.  By  drawing  the 
connecting-rod  and  piston,  any  position  of  the  crank,  found  by 
Fig.  58,  will  give  the  corresponding  position  of  the  piston  in 
the  cylinder. 


149 

CHAPTER   VIII. 
POWER    TRANSMISSION. 

Belts,  Ropes,  and  Gearing. — When  an  engine  is  not  coupled 
directly  to  the  machine  it  is  required  to  drive,  such  as  a  dynamo, 
or,  let  us  say,  to  a  propeller  shaft  on  board  ship,  and  it  is 
necessary  to  transmit  the  power  to  a  machine  some  distance 
away,  such  transmission  is  usually  effected  by  means  of  belts  or 
ropes,  pulleys,  and  steel  shafting.  For  powers  up  to  100  H.P.  flat 
leather  belts  are  generally  used ;  such  belts  can  transmit  up  to 
200  or  250  H.P.,  or  more,  but  ropes  are  usually  preferred  when 
powers  greater  than  100  H.P.  have  to  be  dealt  with. 

The  amount  of  power  a  belt  is  able  to  transmit  depends  upon 
the  width  of  the  belt,  the  speed  at  which  it  runs,  upon  the 
strength  of  the  belt  and  its  fastenings,  and  the  extent  to  which 
the  belt  laps  round  and  grips  the  pulley. 

In  a  well-arranged  horizontal  belt  drive  the  driving  side  is 
underneath,  the  slack  side  being  uppermost,  so  that  the  sag  of 
the  slack  side  causes  the  belt  to  wrap  itself  more  completely 
round  the  pulley  than  would  be  the  case  if  the  driving  side  were 
on  the  top  and  the  slack  side  underneath.  In  the  majority  of 
cases  of  belt  transmission  the  belts  are  placed  at  an  angle. 
A  very  steep  drive — i.e.,  one  in  which  the  belt  is  nearly  vertical 
— should  be  avoided  if  possible,  as  the  more  nearly  the  drive 
approaches  the  vertical  the  greater  the  tendency  of  the  belt  to 
slip.  To  avoid  such  slip  the  belt  requires  to  be  very  tightly 
laced,  and  this  causes  undue  friction  on  the  bearings  of  the 
shaft,  and  consequent  loss  of  power. 

The  power  which  can  be  transmitted  by  leather  belts  can  be 
found  approximately  by  the  following  formula: — * 
TWY 
33,000  " 

*  To  be  strictly  accurate  the  formula  should  take  into  account  the 
exact  extent  of  the  circumference  of  the  pulley  embraced  by  the  belt,  but 
in  ordinary  practice  the  width  of  a  belt  is  never  cut  down  to  such  a  fine 
point  that  calculations  going  into  these  minute  points  need  be  made.  A 
draughtsman  who  would  spend  a  morning  in  making  calculations  of  this 
nature  in  connection  with  the  width  of  a  pulley,  would  be  of  little  use  to 
his  employer.  The  efficiency  of  a  draughtsman  is  usually  reckoned  by  the 
following  formula: — 

where  E  =  efficiency  of  draughtsman. 
g  _  W  x  A  W  =  work  turned  out. 

T       '  A  =  accuracy,  sufficient  for  all  practical  purposes. 

T  =  time  occupied. 


150  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

where  T  =  working  tension  in  pounds  per  inch  of  width. 
W  =  width  of  belt  in  inches. 
V  =  velocity  in  feet  per  minute. 

The  safe  working  tension  of  leather  belts  depends  upon  the 
thickness,  and  as  the  thickness  of  hide  does  not  vary  much, 
additional  thickness  is  given  by  placing  two  or  more  hides 
together.  A  single  belt  is  formed  out  of  a  thick  single  hide  and 
is  about  T3F  inch  thick ;  a  double  belt  consists  of  two  thicknesses 
of  hide  and  may  vary  from  J  to  f  inch  thick.  For  the  purpose 
of  the  above  formula  the  safe  working  tension  of  leather  belts 
may  be  taken  as  follows  : — 

Single  belt  -j\  inch  thick,  T  =  55  Ibs. 
Double  belt  f  inch  thick,  T  =  80  Ibs. 

Let  us  work  out  an  example. 

Example. — What  horse-power  can  be  transmitted  from  a  3-foot  pulley 
running  at  200  revolutions  per  minute,  the  face  of  the  pulley  being 
10  inches  wide  and  the  belt  used,  say,  9  inches  wide?  To  find  the  velocity 
we  multiply  the  circumference  of  the  pulley,  which  is  9*42  feet,  by  the 
number  of  revolutions  per  minute,  9 '42  x  200  =  1,884.  We  will  assume 
that  a  single  belt  is  used,  the  safe  working  tension  of  which  we  read  above 

is  55  Ibs.    The  calculation  then  is55  x.9  x  1<884  =  28'2  H.P.    The  answer 

33, 1)00 
is  28'2  horse-power. 

If  a  double  belt  had  been  used,  or  if  the  velocity  had  been  greater,  the 
belt  would  have  been  capable  of  transmitting  more  horse-power. 

If  we  wish  to  find  the  width  of  belt  required  to  transmit  a 
certain  horse-power  the  formula  transposed  is 


W  = 


H.P.  x  33,000 
TV 


In  cases  where  large  powers  have  to  be  transmitted  at  low 
speeds,  belts  formed  of  leather  links  are  sometimes  used ;  these 
belts  are  more  flexible  than  very  thick  solid  belts ;  also  as  air 
can  get  away  through  the  links  they  grip  a  wide  pulley  better 
than  a  solid  belt,  and  are  considered  capable  of  transmitting 
about  30  per  cent,  more  power.  Centrifugal  force,  however, 
renders  these  belts  unsuitable  for  high  speeds,  on  account  of 
their  great  weight. 

Leather  belts  will  run  satisfactorily  up  to  a  speed  of  about 
3,000  feet  per  minute ;  beyond  this  speed  centrifugal  force 
interferes  somewhat  with  the  gripping  action.  In  cases  where  a 
belt  will  not  transmit  the  required  power,  owing  either  to  the 


POWER    TRANSMISSION.  151 

result  of  centrifugal  force,  or  to  insufficient  width,  the  required 
power  can  usually  be  obtained  by  using  two  belts,  one  over  the 
other ;  this  is  sometimes  called  compounding.  As  the  outer  belt 
travels  a  little  faster  than  the  inner  one  and  is  usually  narrower, 
the  centrifugal  forces  do  not  act  in  the  same  way  on  the  two 
belts  and  a  better  gripping  action  results.  The  power  trans- 
mitted by  two  belts  placed  one  over  the  other  is  greater  than 
would  be  the  case  if  the  two  belts  were  placed  side  by  side  on  a 
very  wide  pulley,  as  apart  from  the  centrifugal  action  one 
cannot  be  sure  that  the  tension  is  the  same  in  the  two  belts 
when  so  placed. 

Leather  belts  when  compounded  have  run  satisfactorily  up  to 
speeds  of  8,000  feet  per  minute. 

Canvas  belting  is  sometimes  used  for  driving  purposes ;  it  is 
very  flexible,  but  its  life  is  not  so  long  as  that  of  a  leather  belt. 
The  ends  of  leather  belts  are  fastened  together  by  laces.  Many 
forms  of  metal  fasteners  are  sold,  but  it  is  doubtful  whether  a 
joint  made  by  them  lasts  as  long  as  a  properly  laced  joint. 

Pulleys  for  driving  belts  are  made  of  cast  iron,  wrought  iron, 
and  are  sometimes  built  of  wood.  If  the  pulley  is  to  be  placed 
at  the  end  of  an  engine  or  other  shaft  it  is  made  solid  and  keyed 
on,  but  the  majority  of  pulleys  are  now  made  in  halves  so  that 
they  may  readily  be  put  on  the  shaft  while  the  latter  is  in 
position.  Wrought-iron  pulleys  are  slightly  more  expensive 
than  cast  iron,  but  are  usually  preferred  on  account  of  their 
lightness  and  strength.  If  the  pulleys  are  to  run  at  a  high 
speed  they  must  be  properly  balanced — i.e.,  no  part  of  the  rim 
must  be  heavier  than  another  unless  exactly  balanced  by  a 
corresponding  weight  on  the  opposite  side.  If  this  balance  is 
neglected  the  pulley  will  run  untruly,  and  may  distort  the  shaft 
or  loosen  the  bearing. 

The  faces  of  pulleys  for  leather  belts,  with  the  exception  of 
fast  and  loose  pulleys,  are  always  "crowned" — i.e.,  slightly  convex; 
if  they  are  not  crowned  the  belt  will  run  off.  A  leather  belt 
always  tends  to  mount  the  highest  part  of  a  pulley  face ;  hence 
if  the  centre  of  the  face  is  the  highest  part,  the  belt  will  run  up 
to  the  centre  of  the  pulley  and  remain  there.  If  the  convexity 
of  a  pulley  is  too  great,  the  centre  portion  of  the  belt  only  grips 
the  surface  and  the  belt  fails  to  transmit  the  power  it  is  capable 
of.  It  has  been  found  in  such  cases  that  by  reducing  the  con- 
vexity of  the  pulley  greatly  increased  power  is  transmitted. 
The  rule  frequently  given  in  text-books  that  the  convexity  of 
a  pulley  should  be  from  J  to  J  inch  for  every  foot  width  of 
pulley,  if  acted  upon,  would  probably  give  rise  to  the  trouble 
referred  to. 


152  MECHANICAL    ENGINEERING   FOR    BEGINNERS. 

In  the  opinion  of  the  late  Mr.  Tullis,  who  probably  had  more 
experience  in  connection  with  belt  driving  than  most  men,  a 
convexity  of  T*F  inch  is  sufficient  for  pulleys  up  to  6  inches  wide, 
and  a  convexity  of  -£%  inch  for  very  wide  pulleys.  If,  however, 
the  shaft  is  vertical  and  the  pulley  horizontal  (an  unusual 
arrangement  except  in  the  case  of  some  machine  tools),  the 
convexity  should  be  doubled. 

By  varying  the  size  of  the  pulleys  the  relative  speed  of  the 
driving  and  driven  shafts  can  be  varied  through  a  wide  range, 
but  it  is  not  good  practice  to  have  a  greater  difference  than  4  to  1 
between  any  two  pulleys.  That  is  to  say,  a  4-foot  pulley  should 
not  drive  one  smaller  than  1  foot  diameter,  or  greater  than  16 
feet  diameter ;  it"  this  ratio  is  exceeded  the  extent  of  the  circum- 
ference of  the  small  pulley  embraced  by  the  belt  or  rope  is  small. 
When  a  large  ratio  must  be  given  the  pulleys  should  be  placed 
as  far  apart  as  possible. 

In  cases  where  the  two  pulleys  cannot  be  placed  a  good 
distance  apart,  and  it  is  necessary  for  the  belt  to  lap  well  round 
the  small  pulley,  th^  belt  is  sometimes  left  rather  slack,  and  a 
third  pulley  is  used  to  press  the  slack  portion  of  the  belt  towards 
the  tight  portion.  This  third  pulley  is  called  a  Jockey  pulley. 
It  is  not  often  employed,  as  it  is  noisy,  and  tends  to  wear  the 
belt  out  quickly. 

The  speed  at  which  a  driven  pulley  of  a  given  size  will  run,  if 
the  diameter  and  speed  of  the  driving  pulley  are  known,  is  found 
as  follows: — 

D  x  S 
-rf-  =  *> 

where  D  =  diameter  of  the  driving  pulley. 
S  =  speed  of  the  driving  pulley. 
d  =  diameter  of  the  driven  pulley. 
s  =  speed  of  the  driven  pulley. 

Example. — Suppose  we  have  a  pulley,  3  feet  in  diameter,  on  a  shaft 
running  at  200  revs,  per  minute,  which  drives  by  belt  a  pulley  2  feet  in 
diameter,  at  what  speed  will  the  latter  run?  By  the  above  formula 
3  x  200  =  600  -f-  2  =  300.  The  answer  is  300  revs,  per  minute 

If  we  know  the  diameter  and  speed  of  the  driving  pulley,  and 
require  to  know  what  diameter  the  driven  pulley  must  be  in 
order  to  make  the  latter  run  at  a  given  speed,  the  formula  is 
merely  transposed  thus : — 

D  x  S 

=  a. 

s 

Example. — Suppose  the  driving  wheel  is  3  feet  in  diameter,  and  runs  at 
200  revs,  per  minute,  what  diameter  must  the  driven  wheel  be  to  give  a 
speed  of  120  revs,  per  minute?  The  calculation  is  3  x  200  -v-  120  =  5  feet. 


POWER    TRANSMISSION. 


153 


To  ensure  a  long  life  for  a  belt,  the  pulleys  should  be  of 
adequate  size.  In  cases  where  the  pulley  must  be  of  small 
diameter  a  thin  belt  should  be  used. 

The  following  are  suitable  thicknesses  of  belts  for  small 
pulleys : — 

Pulley  4  inch  or  less  diameter ;  Belt  ^  inch  thick, 
to    8  inch         „  „  -^         „ 

19  3 

•^          5)  J5  J)      TF  " 

18    ,,  „  „     i        „ 

In  all  cases  where  a  machine  is  driven  by  belt  from  the 
main  shafting,  means  must  be  provided  for  starting  and  stopping 
it  without  arresting  the  progress  of  the  main  shafting.  This  is 
effected  by  means  of  fast  and  loose  pulleys  ;  the  pulley  on  the 
main  shaft  is  a  wide  one,  not  crowned ;  the  machine  is  provided 
with  two  pulleys  side  by  side,  one  of  which  is  free  to  revolve  on 
its  spindle.  When  the  machine  is  at  rest  the  belt  drives  the 
free  pulley ;  when  it  is  desired  to  start  the  machine  the  belt  is 
moved  by  a  fork  and  lever,  called  "  the  striking  gear,"  on  to  the 
fixed  pulley,  and  so  the  machine  is  driven. 

In  many  cases,  as  in  most  lathes,  it  is  usually  more  convenient 
to  place  the  fast  and  loose  pulleys  on  a  shaft  overhead.  The 
countershaft,  as  it  is  called,  has  pulleys  of  different  sizes  corre- 
sponding with  similar  pulleys  on  the  lathe,  called  cone  pulleys; 
this  arrangement  permits  of  the  speed  of  the  lathe  being  varied : 
thus,  when  the  belt  is  on  the  large  pulley  of  the  countershaft,  it 
is  on  the  small  pulley  of  the  lathe,  and  the  latter  runs  fast. 
When  the  belt  is  on  the  small  pulley  of  the  countershaft,  and  on 
the  large  pulley  of  the  lathe,  the  latter  runs  slowly.  The  fork 
of  the  striking  gear  prevents  the  belt  from  running  off  the 
uncrowned  faces  of  the  fast  and  loose  pulleys. 

Rope  Driving. — The  power  transmitted  by  ropes  may  be 
found  from  the  following  figures,  which  err,  if  at  all,  on  the 
safe  side: — 


One  rope     .     . 

S 

1 

U 

H 

6 

7 

If 

2 

inches  in  diameter 

Will  transmit  J 

2 
2-5 

2-5 
3 

4-5 
5 

8 
10 

10 
13 

/  horse-power  if  hemp 
\      ropes  are  used 
{horse  -power  if  cotton 
ropes  are  used 

for  every  1,000  feet  velocity  per  minute. 

154  MECHANICAL    ENGINEERING   FOR    BEGINNERS. 

Example. — Suppose  we  have  a  pulley,  38  inches  in  diameter,  which  has 
grooves  for  ten  J£-inch  ropes,  the  speed  is  470  revs,  per  minute,  what 
power  will  it  transmit?  We  must  first  find  the  velocity:  the  circum- 
ference of  a  38-inch  pulley  is  119*4  inches,  or  9 '95  feet;  we  multiply  this 
by  the  number  of  revolutions,  and  the  result  is  4,676  feet  velocity  per 
minute.  By  the  above  rule  one  I  J-inch  cotton  rope  will  transmit  5  H.P. 
for  every  1,000  feet  velocity;  therefore,  ten  ropes  will  transmit  50  H.P.  for 
every  1,000  feet  velocity.  The  velocity  we  have  found  is  4 '67  thousand 
feet  per  minute,  therefore  4 '67  x  50  =  233 "5.  The  H.P.  transmitted 
is  233-5. 

A  rope  pulley  should  be  not  less  than  30  times  the  diameter 
of  the  rope,  if  a  cotton  rope  is  used;  or  40  times  the  diameter,  if 
a  hemp  rope  is  used.  Thus,  if  we  have  a  pulley  30  inches  in 
diameter  we  must  not  use  cotton  ropes  larger  than  1  inch  in 
diameter,  or  hemp  ropes  larger  than  j  inches  in  diameter.  A 
small  rope  bends  more  easily  than  a  large  one;  therefore,  as 
a  general  rule,  it  is  better  to  have  a  good  many  small  ropes 
than  a  few  of  large  diameter. 

Ropes  may  be  run  up  to  a  speed  of  7,000  feet  per  minute,  but 
a  speed  of  between  4,500  and  5,000  feet  per  minute  is  considered 
the  best.  Cotton  ropes  are  more  expensive  than  hemp  ropes, 
but  as  they  last  much  longer,  they  are  really  cheaper  in  the  long 
fun.  Makers  of  ropes  usually  speak  of  them  by  the  circum 
ference;  thus  a  maker's  3-inch  rope  would  be  one  of  about 
1  inch  diameter.  Ropes  should  not  touch  the  bottom  of  the 
V-shaped  groove  in  the  pulleys,  but  should  wedge  themselves 
against  the  sides.  The  best  angle  for  the  sides  of  the  groove  is 
about  40^  when  the  diameter  of  the  rope  is  over  1  inch,  and  an 
angle  of  about  30°  when  the  rope  is  less  than  1  inch  in  diameter. 

When  the  diameter  of  a  rope  pulley  is  spoken  of,  the  effective 
diameter — i.e.,  the  diameter  of  the  pulley  where  the  ropes  grip 
the  sides — is  meant.  The  ends  of  ropes  are  spliced  together; 
the  rope  makers  send  out  men  specially  qualified  for  doing  this 
work. 

It  has  been  found  in  practice  that  in  cases  where  the  drive 
is  irregular,  as  with  a  gas  engine,  a  steadier  drive  can  be 
obtained  by  placing  the  slack  side  of  the  rope  underneath, 
and  the  driving  side  uppermost,  than  by  the  opposite 
arrangement. 

A  question  which  frequently  occurs  in  laying  out  a  rope  drive 
is  ; — What  is  the  minimum  distance  at  which  the  pulleys  should 
be  placed  apart  1  A  rule  given  by  Mr.  Kenyon  (an  authority  on 
rope  driving)  is  as  follows : — Take  the  difference  between  the 
diameter  of  the  largest  and  of  the  smallest  pulley  and  add  it  to 
one  and  a-half  times  the  diameter  of  the  largest;  the  result 
gives  the  distance  between  centres  of  the  pulleys. 


POWER    TRANSMISSION.  155 

Example. — We  have  a  driving  pulley  4  feet  in  diameter,  and  a  driven 
pulley  1  foot  in  diameter ;  how  near  may  they  be  placed  together  ?  The 
difference  between  the  diameters  of  the  two  pulleys  is  3  feet.  We  add 
this  to  one  and  a-half  times  the  diameter  of  the  large  pulley — 3  +  (1J  x  4) 
=  9  feet.  The  pulleys  should  therefore  be  placed  not  less  than  9  feet 
apart.  If  the  pulleys  had  each  been  4  feet  in  diameter,  the  difference 
between  their  diameters  would  be  nil,  so  that  the  minimum  centres  would 
be  li  x  4  =  6  feet. 

When  ropes  are  used  for  the  transmission  of  power,  ordinary 
fast  and  loose  pulleys  are  useless,  as  the  ropes  cannot  be  passed 
from  one  to  the  other  as  is  possible  with  a  belt.  To  overcome 
this  difficulty  the  pulley  carrying  the  ropes  can  be  made  either 
fast  or  loose  by  means  of  a  clutch.  A  plain  pulley  dog-clutch  is 
one  in  which  the  pulley  is  free  to  revolve  on  the  shaft,  but. 
adjoining  it  is  a  "  dog  "  which  cannot  revolve  on  the  shaft,  but 
is  free  to  slide  along  it.  This  dog  has  projections  which,  when 
it  is  pressed  up  against  the  pulley,  engage  with  corresponding 
projections,  and  causes  the  pulley  to  revolve.  When  the  dog  is 
withdrawn  the  pulley  remains  stationary  and  the  shaft  revolves. 

An  expanding  clutch  is  one  in  which  one  portion  of  the  pulley 
carrying  the  rim  is  free  to  revolve,  while  the  other  portion, 
which  is  fixed  to  the  shaft,  is  arranged  so  that  its  diameter 
can  be  increased  or  diminished.  When  .the  diameter  of  this 
portion  of  the  pulley  .is  increased,  it  grips  the  inside  of  the 
rim  of  the  other  portion,  and  so  compels  it.  to  rotate. 

Another  clutch  is  similar  in  principle  to  the  dog-clutch,  but 
the  dog  is  made  of  large  diameter,  and  both  it  and  the  loose 
portion  are  provided  with  a  large  number  of  steel  wire  bristles 
similar  to  hair  brushes.  When  the  two  portions  of  the  clutch 
are  brought  together  the  bristles  engage  with  one  another,  and 
the  dog  portion  of  the  clutch  compels  the  other  portion  to 
revolve.  The  object  of  this  clutch  is  to  avoid  shock  if  the 
pulley  requires  to  be  coupled  or  uncoupled  while  running.  The 
author's  experience  with  this  form  of  clutch  is  that  it  must  be 
of  very  ample  size  for  the  work,  otherwise  the  wire  bristles  do 
not  last. 

Loss  of  Power  in  Transmission. — The  loss  incurred  in  the 
transmission  of  power  by  means  of  belts,  ropes,  and  shafting  is 
considerable.  It  is  usually  considered  that  a  belt  drive  absorbs 
about  5  per  cent,  of  the  power  transmitted,  and  a  rope  drive 
about  7  per  cent.  In  one  case  which  came  to  the  author's  notice, 
a  steam  engine  engaged  in  driving  a  portion  of  an  engineer's 
machine  shop,  indicated  SO  H.P.  when  all  the  tools  were  working. 
W^hen  none  of  the  tools  were  at  work,  and  the  engine  was  merely 
driving  the  shafting  and  belts,  the  engine  indicated  30  H.P. 
The  power  lost  in  engine  friction  was  probably  about  8  H.P.,  so 


156 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


that  the  remaining  22  H.P.,  or  27J  per  cent,  of  the  power 
developed,  was  spent  in  transmission. 

This  loss  of  power  has  often  raised  the  question  of  electric 
transmission,  but  the  high  first  cost  of  the  dynamo  and  motors 
generally  puts  this  method  of  transmission  out  of  the  question, 
even  if  the  gain  were  shown  to  be  considerable.  In  point  of 
fact,  the  gain  is  not  very  great,  as  only  90  or  92  per  cent,  of  the 
power  put  into  the  dynamo  is  given  out  as  electric  energy,  and 
only  88  to  90  per  cent,  of  the  electrical  energy  is  restored  by  the 
motor  in  the  form  of  mechanical  energy  ;  there  is  consequently  a 
loss  of  about  20  per  cent.,  without  taking  into  account  the  loss  of 
power  in  connection  with  those  tools  which  still  require  fast  and 
loose  pulleys  and  belts.  If  we  assume  that  these  absorb  7J  per 
cent,  of  the  power,  we  are  no  better  off  through  transmitting  the 
power  electrically  than  by  doing  so  through  shafting  and  pulleys. 
It  would  certainly  be  too  expensive,  even  if  it  were  desirable,  to 
fit  a  small  motor  to  every  machine;  the  efficiency  of  small  motors 
is  nothing  like  so  great  as  it  is  in  those  of  large  size.  In  cases, 
however,  where  power  has  to  be  transmitted  to  considerable 
distances,  electric  transmission  is  frequently  advantageous. 

Shafting. — The  shafting  which  transmits  the  power  and 
carries  the  pulleys  is  usually  made  of  mild  steel ;  the  lengths 
are  joined  together  by  cast-iron  couplings. 

The  power  which  may  be  transmitted  by  a  steel  shaft  can  be 
obtained  from  the  following  figures  : — 


li 

If 

2 

2J 

2£ 

3 

4 

5 

6 

\  Dia.  of  shaft, 
/   in  inches. 

6-25 

10 

16 

21 

30 

50 

120 

235 

400 

}  H.P.  per  100 

/  revs. 

The  power  which  a  shaft  will  safely  transmit  varies  directly 
as  the  speed ;  the  powers  given  above  are  for  a  shaft  running  at 
100  revs,  per  minute,  so  that,  if  the  shaft  runs  at  200  revs.,  twice 
the  power  will  be  transmitted.  If  we  wish  to  ascertain  what 
power  a  2|-inch  shaft  running  at  225  revs,  will  transmit,  it  is 
only  necessary  to  multiply  30  by  225  and  divide  by  100 ;  the 
answer  is  67 '5  H.P. 

The  above  figures  will  be  found  to  correspond  very  nearly  with 
the  best  present-day  practice,  but  in  any  extreme  case  common- 
sense  must  be  used.  For  instance,  if  we  wish  to  transmit  6J 
H.P.  through  a  shaft  several  hundred  feet  long  running  at  100 
revs.,  we  should  probably  decide  to  use  a  1  J-inch  or  even  a  2-inch 
shaft  (rather  than  one  1 J  inch  diameter)  for  the  sake  of  stiffness, 


POWER   TRANSMISSION.  157 

and  to  prevent  the  shaft  whipping.  Whereas,  if  the  power  had 
to  be  transmitted  through  a  shaft  a  few  feet  long,  a  1  J-inch  shaft 
would  be  sufficient.  The  strength  of  a  shaft  varies  as  the  cube 
of  its  diameter,  so  that  if  the  reader  wishes  to  ascertain  how 
much  power  can  be  transmitted  through  any  shaft,  the  diameter 
of  which  is  not  given  above,  it  is  easy  for  him  to  do  so.  Suppose 
we  wish  to  know  how  much  power  can  be  transmitted  through 
a  shaft  10  inches  in  diameter,  running  at  100  revs,  per  minute. 
We  see  that  a  6-inch  shaft  will  transmit  400  H.P.  at  this 
speed;  the  cube  of  6  inches  is  216,  and  the  cube  of  10  inches 
is  l,oOO;  we  therefore  have  a  simple  proportion  sum — thus, 
216  :  400  ::  1,«>00  :  x.  The  answer  is  1,850;  a  10-inch  shaft  will 
therefore  transmit  1,850  H.P.  at  100  revs.,  3,700  H.P.  at  200 
revs.,  4,625  H.P.  at  250  revs.,  and  so  on.  The  rule  gives  results 
a  little  on  the  safe  side,  especially  in  the  case  of  large  shafts ;  for 
instance,  the  engineers  of  the  first  turbine  installation  at  Niagara 
provided  a  10-inch  shaft  next  the  turbine  for  transmitting  5,000 
H.P.  at  250  revs.,  whereas  by  our  rule  we  should  only  have 
allowed  4,025  H.P.,  or  have  made  the  shaft  10J  inches  diameter. 
However,  the  shaft  journals  some  distance  away  from  the 
turbines  were  made  1 1  inches  diameter,  probably  for  the  sake  of 
stiffness. 

The  figures  given  apply  to  shafts  which  are  subject  to  torsion 
in  one  direction  only,  and  the  reader  is  warned  that  he  must  not 
apply  the  rule  to  propeller  shafts  or  to  the  crank-shafts  of  steam 
and  gas  engines.  These  shafts  are  subject  to  shock  and  stresses 
which  are  not  easily  calculated ;  the  only  safe  guide  as  to  the 
right  proportions  of  a  crank-shaft  is  actual  experience.  A  well- 
known  firm  of  engineers,  making  a  very  successful  gas  engine  of 
large  powers,  has  found  it  necessary,  on  account  of  breakages,, 
continually  to  increase  the  diameter  of  its  crank-shafts  beyond 
the  sizes  which  calculation  would  appear  to  render  necessary, 
until  the  crank-shafts  are  now  half  the  diameter  of  the  piston  I 
A  somewhat  similar  experience  befell  a  firm  of  high-speed  engine 
makers,  who  many  years  ago  suffered  from  broken  crank-shafts. 
The  directors  of  this  firm  were  told  by  the  work's  manager  that 
the  shafts  were  too  weak,  and  that  they  ought  to  be  one-quarter 
the  diameter  of  the  L.P.  piston.  The  manager  was  told  that 
such  a  rule  of  thumb  was  perfectly  ridiculous,  and  that  the 
stresses  could  be  calculated  without  any  difficulty.  After  many 
breakages,  further  calculations  were  made,  enormous  factors  of 
safety  being  apparently  allowed,  the  shafts  were  strengthened, 
but  still  they  broke.  The  shafts  were  again  still  further- 
strengthened,  and  finally  the  breakages  ceased;  but,  curiously 
enough,  the  dimensions  of  nearly  all  the  steam-engine  shafts  now 


158  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

made  by  this  firm  approximate  very  nearly  indeed  to  the  works' 
manager's  despised  rule. 

On  the  face  of  it,  a  rule  which  does  not  take  into  account  the 
mean  pressures,  inertia  of  working  parts,  length  of  stroke,  &c., 
does  look  ridiculous,  but,  upon  going  more  closely  into  the 
matter,  it  will  be  found  that  one  factor  is  probably  counter- 
balanced by  another ;  for  instance,  the  L.P.  piston  of  a  non- 
condensing  engine,  having  a  late  cut-off,  may  have  a  much 
greater  mean  pressure  upon  it  than  the  L.P.  piston  of  a  con- 
densing engine  having  a  fairly  early  cut-off,  but  the  former  will 
have  a  better  cushion  to  absorb  the  inertia  of  the  working  parts 
than  the  latter.  Again,  at  first  sight,  one  would  say  that  a 
slow-speed  engine,  the  L.P.  cylinder  of  which  is  20  inches  in 
diameter  by  30  inches  stroke,  would  require  a  larger  crank-shaft 
than  an  engine  having  the  cylinder  of  the  same  diameter,  but 
with  a  10-inch  stroke,  as  not  only  is  the  crank  of  the  former 
engine  three  times  longer  than  that  of  the  short-stroke  engine, 
and  the  torque  on  the  shaft  correspondingly  greater,  but  the 
weight  of  the  parts  of  the  long-stroke  engine  will  also  be  greater, 
owing  to  the  greater  length  of  connecting-rod.  Against  these 
considerations  must  be  set  the  fact  that  the  rapid  alternations  of 
stress  in  the  crank-shaft  of  an  engine,  running  at  the  high  speed 
which  would  be  expected  from  an  engine  having  a  20-inch  by 
10-inch  L.P.  cylinder,  are  much  more  punishing  to  the  shaft 
than  any  stress  set  up  in  the  shaft  of  the  slow-speed  engine. 
As  a  fact,  in  actual  practice,  the  crank-shafts  of  long-stroke, 
slow-speed  steam  engines  are  usually  made  about  one-fifth  the 
diameter  of  the  L.P.  cylinder,  while  the  crank-shafts  of  short- 
stroke,  high-speed  engines  require  to  be  made  about  one-fourth 
the  diameter  of  the  L.P.  cylinder,  if  they  are  to  have  a  reason- 
able life. 

Bearings. — The  bearings  which  carry  the  shafting  are  called 
plummer  blocks ;  they  contain  a  top  and  bottom  brass  which  can 
be  renewed  when  much  worn.  The  plummer  blocks  are  usually 
carried  on  cast-iron  brackets ;  when  suspended  from  a  joist  or 
beam  the  brackets  which  carry  the  plummer  blocks  are  called 
hangers.  It  is  important  that  all  plummer  blocks  should  be 
accurately  aligned,  and  that  the  brasses  should  be  properly 
lubricated  in  order  to  minimise  friction  as  much  as  possible. 
Hangers  are  sometimes  made  with  adjustable  devices,  by  means 
of  which  the  bearing  may  be  raised,  lowered,  or  made  to  swivel. 

Gearing.— Before  the  advent  of  rope  driving,  it  was  usual  to 
transmit  large  powers  by  means  of  gearing.  For  instance,  a 
large  mill  engine  would  transmit  by  gearing  its  power  to  a 
vertical  shaft  running  the  whole  height  of  the  mill ;  on  each 


POWER    TRANSMISSION.  159 

floor  the  power  would  be  transmitted  to  horizontal  shafts  by 
means  of  bevel  wheels ;  the  horizontal  shafts  would  then  drive 
the  machinery  by  belts. 

The  loss  of  power  in  transmission  by  gearing  is  less  than  in 
transmission  by  ropes,  but  gearing  is  noisy  and  cumbrous,  and  a 
broken  toothed  wheel  may  involve  a  serious  stoppage  of  the 
mill.  Transmission  of  large  powers  by  gearing  is  now  seldom 
resorted  to,  except  in  cases  where  large  powers  have  to  be 
transmitted  at  low  speeds,  as  in  steel  rolling  mills. 

Toothed  gearing  is,  however,  very  useful  in  transforming 
small  powers  at  high  speeds  to  great  powers  at  reduced  speeds, 
as  in  the  case  of  cranes.  Thus,  if  a  small  toothed  wheel  or 
pinion,  2  inches  in  diameter,  drives  a  toothed  wheel,  20  inches 
in  diameter,  the  latter  will  be  able  to  raise  by  a  drum  a  weight 
ten  times  greater  than  would  be  possible  if  the  weight  were 
being  lifted  by  a  drum  fixed  directly  to  the  2-inch  wheel.  The 
weight  will,  however,  be  lifted  ten  times  more  slowly,  so  that 
what  is  gained  in  power  is  lost  in  speed. 

It  is  essential  that  the  teeth  of  gear  wheels  should  be  machine 
cut  and  accurately  formed,  if  the  gear  is  to  work  quietly  and  to 
waste  little  power. 

Toothed  wheels  are  called  spur  wheels  when  their  shafts  are 
parallel  and  they  drive  in  the  same  plane.  When  a  small  wheel 
drives  a  large  one  the  small  wheel  is  called  a  pinion. 

When  one  wheel  drives  another  at  right  angles  to  it,  or  at  an 
angle  slightly  greater  or  less  than  a  right  angle,  the  wheels  are 
called  bevel  wheels. 

When  the  sides  of  a  wheel  are  carried  up  so  as  to  support  the 
ends  of  the  teeth,  the  wheel  is  said  to  have  shrouded  teeth.  But 
one  wheel  only  out  of  a  pair  can  have  shrouded  teeth  if  carried 
up  to  the  top  of  the  teeth.  Both  wheels  can  have  shrouded 
teeth  if  the  shrouding  ends  just  below  the  pitch  line.  A  wheel 
with  shrouded  teeth  is  stronger  than  one  with  plain  teeth,  but 
the  teeth  cannot  be  machine  cut. 

Helical  wheels  are  those  in  which  the  teeth,  instead  of  running 
straight  across  the  face,  are  placed  so  that  every  tooth  forms  two 
sides  of  a  triangle,  the  apex  of  the  triangle  being  at  the  centre 
of  the  face.  The  effect  of  this  is  to  give  more  surface,  so  that  a 
wheel  of  a  given  width  with  helical  teeth  will  transmit  more 
power  than  one  with  plain  straight  teeth,  and  the  action  is 
smoother.  Teeth  of  this  form  cannot  be  machine  cut  unless  the 
two  halves  of  the  teeth  are  separated  by  a  small  space. 

Worm  Gearing. — Fig.  60  shows  a  worm  and  wheel;  the 
worm  has  a  single  spiral,  or  thread,  with  a  1-inch  pitch — i.e., 
1  revolution  of  the  spiral  will  move  forward  the  teeth  engaged 


160 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


with  it  1  inch.     The  wheel  has  twenty  teeth  of  the  same  pitch, 
so   that  it  will  take  20  revolutions  of  the  worm  to  make  the 


Fig.  60.— Worm  wheel. 


Fig.  61.— Worm  wheel. 

wheel  revolve  once.     With  a  single  spiral  it  is  not  practicable  to 
effect  a  reduction  of  speed  greater  than  15  to  1,  and  20  or  25  to 


POWER   TRANSMISSION.  161 

1  is  a  better  proportion.  Such  a  reduction  is  too  great  for  most 
purposes. 

Fig.  61  shows  what  the  effect  would  be  of  a  3  to  1  reduction 
with  an  ordinary  worm.  The  pitch  of  the  worm  would  have  to 
be  very  great,  so  that,  for  every  revolution,  it  would  turn  the 
wheel  through  one-third  of  a  revolution.  The  result  would  be 
that  the  worm  would  drive  one  of  the  teeth  until  it  was  out  of 
mesh,  but  the  next  tooth  would  not  then  be  far  enough  round  to 
engage.  The  difficulty  can,  however,  be  got  over  by  putting 
more  spirals  on  the  worm,  as  shown  by  dotted  lines,  and  making 
a  corresponding  addition  to  the  number  of  teeth  on  the  wheel. 
The  pitch  of  the  spirals  would  remain  the  same,  and  the  reduc- 
tion of  speed  would  also  remain  the  same. 

With  the  ordinary  worm  and  wheel,  as  shown  by  Fig.  60,  the 
wheel  will  not  drive  the  worm  ;  but,  with  a  worm  having  a  very 
coarse  pitch,  the  wheel  will  drive  the  worm  and  vice  versd. 
Worms  with  a  coarse  pitch  and  several  spirals  are  used  in  motor 
cars  where  the  worm  drive  is  employed,  otherwise  the  car  could 
not  move  without  the  engine  being  turned  by  hand  or  being  run 
by  petrol.  With  the  worm  drive  it  is  important  to  make  the 
effective  contact  surfaces  as  large  as  possible,  so  as  to  get  a  large 
area  of  lubrication. 

Skew  wheels  are  constructed  on  the  same  principle — i.e.,  a 
worm  having  many  spirals,  or,  rather,  parts  of  spirals — as  the 
skew  wheel  is  not  sufficiently  wide  for  a  single  thread  to  run 
right  round  it,  as  in  the  case  of  a  worm;  this,  of  course,  is 
immaterial  so  long  as  the  pitch  is  right. 

Gear  wheels,  where  great  strength  is  required,  are  made  of 
steel,  and  in  high-class  work  the  teeth  are  cut  out  of  the  solid. 
In  large  slow-running  wheels  the  teeth  are  sometimes  cast  of 
the  desired  shape  and  trimmed  up  by  hand.  Toothed  wheels 
for  machine  tools  are  usually  made  of  cast  iron  and  carefully 
machined.  Such  wheels  work  more  smoothly  than  steel  wheels. 

Within  recent  years  raw-hide  pinions  have  been  introduced  to 
get  over  the  noise  and  jar  caused  by  a  pair  of  wheels  running  at 
high  speeds ;  they  appear  to  answer  admirably.  Before  these 
were  introduced,  the  author  was  present  at  an  attempt  made  to 
drive  by  gearing  a  dynamo  which  absorbed  about  60  horse-power 
and  required  to  run  at  900  revolutions  per  minute,  this  speed 
being,  of  course,  too  high  to  admit  of  coupling  the  dynamo  shaft 
directly  to  the  engine  shaft.  The  engine  ran  at  380  revolutions 
per  minute  and  drove  the  dynamo  through  a  pair  of  machine-cut 
steel  wheels,  but  the  noise  made  was  so  appalling  that  this 
method  of  drive  had  to  be  abandoned. 

A  good  method  of  transmitting  moderate  powers  at  fairly  high 

11 


162 


MECHANICAL    ENGINEERING    FOR   BEGINNERS. 


speeds  in  cases  where  belts  are  not  admissible,  and  gearing  is  too 
noisy,  is  by  the  Hans  Reynolds  chain,  shown  by  Fig.  62.  With 
this  chain  a  perfectly  vertical  drive  is  permissible.  At  a  large 
engineering  works  in  the  Midlands,  each  main  line  of  shafting 
running  down  the  works  is  driven  by  this  chain  from  a  motor 
placed  directly  underneath  the  shafting.  The  motors  run  at  a 
speed  considerably  higher  than  that  of  the  shafting. 


Fig.  62. — Hans  Reynolds'  silent  chain. 

With  regard  to  the  horse-power  transmitted  by  gearing,  it  is 
not  possible  to  give  a  simple  rule,  as  in  the  case  of  transmission 
by  belts  and  ropes.  A  formula  often  used,  and  which  was 
originally  published  by  Messrs.  Musgrave,  is  as  follows  : — 

H.P.  =  P2  x  B  x.  V^  1,000  for  cast  iron, 
H.P.  =  P2  x  B  x  V  -r    625       „        steel ; 


POWER   TRANSMISSION.  163 

where  P  =  circumferential  pitch  of  teeth  in  inches. 
B  =  breadth  of  wheel  in  inches. 
V  =  velocity  of  pitch  line  in  feet  per  minute. 

This  formula,  however  suitable  for  toothed  wheels  of  fairly 
large  size,  is  not  suitable  for  small  wheels  —  i.e.,  in  cases  where 
the  pitch  is  1  inch  or  less.  Let  us  see  how  it  applies  to  a  small 
toothed  wheel  used  in  the  gear-box  of  a  15-H.P.  motor  car. 
The  wheel  we  will  take  has  a  6f-inch-diameter  pitch  circle  ;  it 
has  twenty  teeth  of  1-inch  pitch,  its  breadth  is  1  J  inches,  and  the 
velocity  of  the  pitch  line  is  1,500  feet. 

12xl-2oxl,500 

-625~  H-P" 

so  that  by  the  formula  the  wheel  would  only  transmit  3  H.P. 

In  actual  practice  the  wheel  transmits  15  H.P.  or  more.  An 
empirical  formula,  given  by  Mr.  Box,  for  the  strength  of  teeth  is 
.as  follows  :  — 

S  =  P  x  W  x  350, 

where     S  =  safe  load  on  one  tooth  in  Ibs. 
P  =  pitch  of  wheel  in  inches. 
W  =  width  of  tooth         „ 

If  we  apply  this  to  the  wheel  under  consideration,  we  get 
1  x  1-25  x  350  =  437  Ibs.  safe  load  on  one  tooth.  Let  us  see 

.,.       .,,  437  Ibs.  x  1,500  feet       -  n  0  ,,  „ 

what  power  this  will  give  us.  —        '  -  =  19  -8  H.P., 


so  that,  by  this  rule,  the  wheel  which  is  used  on  a  15-H.P.  motor 
car  will  safely  transmit  19*8  H.P.  This  formula  is  unsuitable  for 
wheels  in  which  the  pitch  of  the  teeth  is  much  greater  than  1  inch. 

A  beginner,  for  whom  this  book  is  intended,  is  hardly  likely  to 
be  called  upon  to  design  gear  wheels  for  some  time,  and,  as  a 
fact,  there  are  in  most  drawing  offices  where  such  wheels  need  to 
be  designed  some  available  data  as  to  sizes,  strengths,  &c.  ;  such 
data,  if  intelligently  used,  are  of  far  more  value  than  any  empirical 
formulae.  If  in  doubt  as  to  the  strength  of  a  gear  wheel,  it  is  a 
good  plan  to  assume  that  one  tooth  must  be  sufficiently  strong 
to  transmit  the  whole  of  the  power,  and  to  see  what  pressure  in 
pounds  will  come  upon  it.  This  pressure  will,  of  course,  be 
found  by  the  following  formula  :  — 

H.P.  x  33,000 
V        ~; 
where  V  is  the  velocity  in  feet  per  minute  of  pitch  circle. 

Knowing  the  section  of  the  tooth,  it  is  not  difficult  to  form  an 
opinion  as  to  whether  it  is  strong  enough  to  bear  the  load. 


165 


CHAPTER  IX. 
CONDENSING  PLANT. 

CONDENSERS,  although  differing  in  type,  are  all  designed  with 
the  same  object — viz.,  to  extract  from  the  steam  the  heat 
remaining  in  it  after  the  former  has  done  its  work,  so  that  it 
condenses  and  a  vacuum  is  formed,  thus  relieving  the  engine  or 
turbine  from  the  necessity  of  discharging  the  steam  against 
atmospheric  pressure.  Means  must  be  provided  for  getting  rid 
of  the  condensed  steam  without  permitting  air  to  get  into  the 
condenser,  and  so  impair  the  vacuum. 

The  condensers  most  frequently  used  are  either  jet  or  surface 
condensers.  In  an  ordinary  jet  condenser  the  cooling  water  is 
admitted  in  the  form  of  a  jet  or  spray,  as  shown  by  Fig.  63. 
The  exhaust  steam  from  the  engine,  coming  into  contact  with 
this  spray  of  cool  water,  immediately  condenses,  and  a  vacuum 
is  created.  When  the  vacuum  is  once  formed,  the  condenser 
will  continue  to  draw  in  the  injection  water  (owing,  of  course, 
to  the  pressure  of  the  atmosphere  on  the  surface  of  the  water 
outside),  so  that,  unless  the  water  has  to  be  lifted  more  than  10 
or  12  feet,  a  pump  to  supply  the  condenser  with  injection  water 
is  not  required. 

With  a  jet  condenser,  in  which  the  water  mixes  so  intimately 
with  the  steam,  a  smaller  quantity  of  water  is  required  to  con- 
dense the  steam  than  is  necessary  in  a  surface  condenser  where 
the  water  has  to  effect  its  cooling  action  through  tubes ;  but,  on 
the  other  hand,  if  it  is  desired  to  use  the  condensed  steam  over 
again  for  feeding  the  boiler,  it  is  necessary  for  the  cooling  water, 
as  well  as  the  feed  water,  to  be  free  from  impurities  which  may 
be  injurious  to  the  boiler,  for,  as  we  have  said,  the  injection 
water  mixes  with  the  exhaust  steam. 

The  amount  of  injection  or  cooling  water  required  for  a  jet 
condenser  is  about  25  or  30  times  the  weight  of  the  steam  to  be 
condensed.  The  size  of  a  jet  condenser  is  not  of  great  im- 
portance ;  it  is  usually  made  about  three-quarters  the  capacity  of 
the  L.P.  cylinder.  The  shape,  too,  is  of  but  small  importance. 

With  a  jet  condenser  fixed  at,  or  below,  the  level  of  the  engine, 
a  pump  is  required  to  remove  the  condensed  steam,  cooling  water, 
and  a  certain  amount  of  vapour  from  the  condenser;  such  a  pump 


166 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


is  called  an  air  pump,  as  it  prevents  the  air  from  getting  into  the 
condenser,  also  to  distinguish  it  from  a  circulating  pump. 


Injection 
Regulating 
Valve 


To  Mr 

Pump 


Fig  63. — Jet  condenser. 

An  air  pump  is  shown  by  Fig.  64.  The  pump  is  shown  with 
head  valves,  bucket  valves,  and  foot  valves,  but  in  practice  the 
foot  valves  are  now  often  dispensed  with.  The  Edwards  air 


CONDENSING    PLANT. 


167 


pump,  which  has  neither  bucket  valves  nor  foot  valves,  will  be 
described  later. 

Surface  Condensing  Plant. — In  a  surface  condenser  the 
cooling  water  is  kept  separate  from  the  steam  which  it  has  to 
condense.  The  water  is  passed  through  a  large  number  of  tubes, 
usually  about  f  inch  outside  diameter,  made  of  Muntz  metal  or 
of  an  alloy  composed  of  70  per  cent,  copper  and  30  per  cent.  zinc. 
A  surface  condenser  is  shown  by  Fig.  65.  A  condenser  of  this 


•  Access  Doer. 
Fig.  64. — Air  pump. 

type  is  always  used  on  board  ship,  where  it  is  necessary  to  use 
the  condensed  steam  over  and  over  again  for  feeding  the  boilers, 
and  where  it  is  inadmissible  to  mix  sea  water  with  the  feed 
water.  A  surface  condenser  is  also  used  on  land  in  cases  where 
the  cooling  water  is  not  suitable  for  use  in  the  boilers. 

With  a  surface  condenser  the  air  pump  may  be  considerably 
smaller  than  with  a  jet  condenser,  as  it  has  to  deal  with  the 
condensed  steam  and  vapour  only;  a  separate  pump,  called  a 
circulating  pump,  is  used  for  circulating  the  water  through  the 


168 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


tubes.  The  circulating  pump  may  be  similar  to  the  air  pump 
shown  by  Fig.  64 ;  but  pumps  of  the  centrifugal  type,  in  which 
there  are  no  valves,  are  frequently  used.  In  a  centrifugal  pump 
(illustrated  in  a  later  chapter)  there  is  an  impeller,  which  is 
rotated  at  a  rapid  rate,  and  imparts  sufficient  motion  to  the  water 
to  make  it  travel  through  the  tubes  of  the  condenser. 

Extent  of  Cooling  Surface. — In  a  surface  condenser  there 
should  be  1  square  foot  of  cooling  surface  for  every  9  to  10  Ibs. 
of  steam  to  be  condensed  per  hour,  assuming  the  temperature  of 
the  cooling  water  to  be  about  60°  to  70°  F.  In  the  case  of 
condensing  plant  for  a  steam  turbine  where  it  is  desirable  to  get 


Circulatinq 

Water 

Discharge 


Circulating 
Water  Inlet 


Discharge  to 
Air  Pump. 


Fig.  65. — Surface  condenser. 


an  extremely  good  vacuum,  a  square  foot  of  cooling  surface  is 
frequently  provided  for  every  6  to  8  Ibs.  of  steam  to  be  con- 
densed. 

The  tubes  of  a  surface  condenser  should  be  of  a  length  not 
exceeding  12  feet,  unless  supported  in  the  centre.  Each  tube  is 
free  to  slide  in  its  hole  in  the  end  plate,  the  joint  being  made 
by  a  small  brass  ferrule  and  cotton  rope  packing,  as  shown  by 
Fig.  66.  If  the  tubes  are  screwed  or  expanded  into  the  end 
plates  they  are  not  able  to  expand  or  contract,  and  the  condenser 
does  not  remain  air  tight. 

It  has  recently  been  found  that  by  dividing  a  surface  condenser 


CONDENSING    PLANT. 


169 


IS 


into  sections  horizontally  and  draining  away  the  water  from  each 
section,  a  smaller   cooling  surface  is   equally   effective.      It  : 
believed  that  a  thick  film  of  water  hangs 
round  the  tubes,  and  prevents  the  conduc- 
tion of  heat  through  them.     If  the  water 
is  drained  away  from    the  upper  rows  of 
tubes   as   it    is   formed,  the    lower   tubes 
remain   fairly    dry,    and    are   much   more 
effective. 

Amount  of  Cooling  Water  Required. 
— The  amount  of  cooling  water  required 
in  connection  with  a  surface  condenser  is 
variously  stated  in  engineering  pocket- 
books  and  text-books  to  be  from  thirty 
to  seventy  times  the  amount  of  feed  water. 

The  amount  of  cooling  water  required  depends  on  its  tempera- 
ture, but  the  following  table,  showing  the  actual  vacua  obtained 
with  different  quantities  of  feed  water  at  a  temperature  of  65°  F. 
may  be  useful.  The  table  has  been  prepared  from  curves  given 
by  Mr.  Allen  in  his  paper  upon  condensing  plants  read  before 
the  Institution  of  Civil  Engineers  in  1905. 

The  curves  themselves  were  plotted  from  a  very  large  number 
of  experiments  carried  out  by  Mr.  Allen : — 


Fig.  66. — Tube  end  and 
ferrule,  half  size. 


Cooling  Surface  =  1  Square  Foot  for 
5  Lbs.  of  Steam  Condensed. 

Cooling  Surface  =  1  Square  Foot  for 
10  Lbs.  of  Steam  Condensed. 

Amount  of  Cooling 
Water. 

Vacuum. 

Amount  of  Cooling 
Water. 

Vacuum. 

40  times  feed. 
50 
60 
70 

27-2" 
2*7-1" 
28-0" 
28-25" 

40  times  feed. 
50 
60 
70 

26-75" 
27-25" 
2775" 
28-0" 

From  the  above  figures  it  will  be  seen  that  for  a  vacuum  of 
26  inches  or  27  inches,  forty  times  the  amount  of  the  feed  is 
sufficient,  but  if  very  high  vacua  are  desired  the  amount  of 
cooling  water  must  be  increased,  or  water  of  a  lower  temperature 
used. 

The  air  pump  in  the  experiments  in  question  had  a  capacity 
of  -75  cubic  foot  per  pound  of  steam  condensed.  Temperature 
of  cooling  water  65°  F.,  Barometer  29 -9. 

Capacity  of  Air  Pump. — The  capacity  of  the  air  pump— i.e., 
the  volume  swept  by  the  bucket  multiplied  by  the  number  of 


170 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


strokes  per  minute — should  be  from  -75  to  1  cubic  foot  per  pound 
of  steam  to  be  condensed.  With  some  surface  condensing  plants 
a  pump  capacity  of  1-5  cubic  feet  per  pound  of  steam  to  be 
condensed  has  been  allowed  ;  it  is,  however,  useless  to  provide 
an  air  pump  of  very  large  capacity,  unless  the  temperature  of  the 
cooling  water  is  fairly  low,  and  the  condenser  has  adequate 
cooling  surface.  For  example,  assuming  the  condenser  is  unable 
to  reduce  the  temperature  of  the  exhaust  steam  below  105°  F., 
no  air  pump,  however  large,  can  give  a  better  vacuum  than 
27 '7  inches  (see  the  subjoined  table),  for  at  this  temperature  and 
pressure  water  turns  into  steam.  If  an  attempt  is  made  to  get 
a  higher  vacuum  than  this  by  means  of  the  air  pump,  not  only 
will  the  exhaust  steam  not  condense,  but  any  water  lying  in  the 
condenser  will  vaporise,  and,  as  the  pump  will  be  quite  unable 
to  cope  with  such  an  enormous  volume  of  steam,  the  vacuum 
will  fall  to  a  point  at  which  steam  will  condense  at  a  temperature 
of  105°  F. 

TABLE    XYI. 


Degrees 
Fahr. 

Vacuum  in 
Inches  when 
Atmospheric 
Pressure 
=  14-7  Lbs. 

Absolute 
Pressure  in 
Inches  of 
Mercury. 

Absolute 
Pressure. 

Total  Heat  in 
1  Lb.  of  Steam 
from  32°  F. 

Volume  of 
1  Lb.  of 
Steam. 

Lbs.  per  sq.  in. 

B.T.U. 

Cubic  feet. 

32 

29-7 

•181 

•089 

1091-2 

3,226 

50 

29-6 

•362 

•178 

109tr6 

1,695 

60 

29-4 

•517 

•254 

1099-7 

1,220 

70 

29-2 

•733 

•360 

1102-8 

877 

80 

28-9 

1-024 

•503 

1105-8 

641 

90 

28-5 

1-410 

•693 

1108-9 

549 

100 

28-0 

1-917 

942 

1111-9 

353 

105 

27-7 

2-229 

1-095 

1113-4 

307 

From  this  table  it  will  be  seen  that,  in  order  to  obtain  a 
vacuum  of  28  inches,  the  temperature  of  the  steam  must  be 
reduced  below  100°  F.  To  obtain  a  vacuum  of  29-6  the  steam 
must  be  cooled  below  50°  F. 

If  the  atmospheric  pressure  is  higher  than  14/7  Ibs.  (or  29-95 
inches  of  mercury)  then  a  slightly  better  vacuum  can  be  obtained 
with  the  above-mentioned  temperatures.  The  vacuum  it  is 
possible  to  obtain  theoretically  with  given  temperatures,  and 
with  the  barometer  standing  higher  or  lower  than  29-95  inches 
can  be  ascertained  by  deducting  the  pressure  given  in  inches  of 
mercury  (column  3)  corresponding  with  the  temperature  of  the 
steam,  from  the  height  of  the  barometer  in  inches  of  mercury. 


CONDENSING    PLANT.  171 

Some  engineers  employ  two  air  pumps  with  a  surface  condenser, 
one  a  dry  air  pump  for  carrying  away  the  vapour,  and  the 
other  a  wet  pump  for  removing  the  water.  The  former  is 
connected  to  the  top  of  the  condenser  (the  vapour  preferably 
being  cooled  before  admission  to  the  pump),  and  the  latter  to  the 
lowest  part  of  the  condenser.  It  is  somewhat  doubtful,  however, 
whether  the  advantage  gained  outweighs  the  additional  cost 
and  complication  involved. 

Vacuum  Augmenter. — In  order  to  assist  the  air  pumps  to 
deal  with  vapour  Mr.  Parsons  has  introduced  what  he  calls  a 
vacuum  augmenter.  This  is  a  small  apparatus  fitted  between 
the  condenser  and  the  air  pump,  and  through  which  a  jet  of  live 
steam  is  blown ;  this  jet  draws  out  considerable  quantities  of 
vapour  from  the  pipe  to  which  it  is  fitted,  compresses  it,  and 
delivers  it  to  the  air  pumps. 

In  striving  for  a  high  vacuum  there  is  one  point  which  must 
not  be  overlooked,  it  is  this — It  is  quite  useless  to  have  a  very 
high  vacuum  in  the  condenser  if,  owing  to  the  want  of  area  of 
the  exhaust  ports,  a  correspondingly  high  vacuum  is  not  obtained 
in  the  cylinder  of  the  engine.  The  reader  will  see  from  the 
last  column  of  Table  xvi.  how  enormous  is  the  volume  of  a 
pound  of  steam  at  very  low  pressure  (or  high  vacuum),  and  will 
realise  how  difficult  it  must  be  to  get  such  a  volume  of  steam  out 
of  a  cylinder  through  its  ports. 

In  Lancashire  it  has  been  found  from  experience  that  a 
vacuum  of  26  inches  gives  the  most  economical  results.  The 
explanation  doubtless  is  that,  if  a  higher  vacuum  is  obtained  in 
the  condenser,  the  exhaust  steam  must  be  cooled  down  to  a  much 
greater  extent,  and  the  temperature  of  the  condensed  steam  which 
is  fed  into  the  boiler  is  correspondingly  reduced.  This  reduced 
temperature  of  the  boiler  feed  probably  neutralises  the  gain  due 
to  a  slightly  better  vacuum  in  the  engine. 

Corrosion  of  Condenser  Tubes. — The  galvanic  action  set 
up  by  Muntz  metal  tubes,  brass  tube  plates,  and  the  cast-iron 
shell  tends  to  make  the  latter  corrode.  This  in  itself  is  not 
very  harmful  if  the  shell  is  made  fairly  thick  in  the  first  place, 
but  if  a  piece  of  rusty  iron  lodges  in  one  of  the  tubes  it  quickly 
corrodes  its  way  right  through.  To  overcome  this  source  of 
trouble,  Mr.  Edwards  advocates  increasing  the  speed  at  which 
the  water  travels  through  the  tubes,  so  as  to  sweep  them  more 
effectually.  The  usual  speed  is  about  300  feet  per  minute ;  by 
greatly  increasing  this  velocity  Mr.  Edwards  claims  that  he 
has  effected  an  improvement  in  the  life  of  the  tubes.  Coating 
the  inside  of  the  condenser  body  with  a  wash  of  cement  has 
proved  very  useful  in  preventing  corrosion. 


172 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


Edwards'  Air  Pump. — A  pump  which  has  come  very  largely 
into  use  during  the  last  few  years  is  Edwards'  air  pump,  as 
shown  by  Figs.  67  and  68.  This  air  pump  differs  from  those  of 
the  older  pattern,  in  that  it  has  no  bucket  valves  or  foot  valves, 


and  runs   at   a   considerably  higher  speed   than  was   formerly 
considered  practicable. 

The  pump  is  placed  below  the  level  of  the  condenser,  so  that 
the   condensed   steam  flows  continuously  by  gravity  from  the 


CONDENSING    PLANT.  173 

condenser  to  the  base  of  the  pump.  On  the  downward  stroke 
a  vacuum  is  created  between  the  bucket  and  the  head  valves, 
and  at  the  bottom  of  the  stroke  the  bucket  uncovers  a  row  of 
ports,  so  that  the  vapour  from  the  condenser  enters  the  barrel. 
By  its  conical  shape  the  bucket  projects  the  water  which  was 
lying  in  the  base  of  the  pump  through  the  ports  into  the  pump 
barrel ;  the  water  is  projected  with  considerable  force,  and  it 
also  entrains  a  certain  amount  of  vapour.  On  the  up  stroke  the 
bucket  closes  the  ports  and  sweeps  out  the  water  and  vapour 
through  the  head  valves. 

This  pump  has  many  advantages ;  in  the  first  place,  the  bucket 
valves  and  foot  valves,  which,  in  the  old  form  of  pump,  were 
necessarily  rather  inaccessible,  are  eliminated.  In  the  second 
place,  when  the  bucket  uncovers  the  ports  the  vapour  has  free 
entry  to  the  barrel,  whereas,  in  the  old  form  of  pump,  a  certain 
pressure  in  the  condenser  was  necessary  to  open  the  valves.  In 
the  third  place,  owing  to  the  high  speed  at  which  it  is  possible 
to  run  the  pump,  it  deals  with  small  quantities  of  water  at  a 
time,  and  runs  very  smoothly  and  with  freedom  from  shock. 
There  is  a  large  inspection  door,  shown  at  the  top  right-hand 
side  of  the  illustration,  which  gives  access  to  the  head  valves. 

Evaporative  Condenser. — In  cases  where  water  for  con- 
densing purposes  is  scarce,  or  has  to  be  paid  for,  and  the  amount 
of  steam  to  be  condensed  is  comparatively  small,  an  evaporative 
condenser  is  sometimes  employed.  This  form  of  condenser 
consists  of  a  range  of  pipes  having  external  gills,  through  which 
pipes  the  steam  is  passed.  Water  is  allowed  to  trickle  on  to 
them,  and  the  water  is  evaporated,  thus  extracting  a  considerable 
amount  of  heat  from  the  pipes.  By  this  system,  instead  of  30 
to  40  times  the  amount  of  feed-water  being  required  for  con- 
densing purposes,  an  amount  of  water  equivalent  only  to  the 
feed  is  required.  The  remarks  in  connection  with  power  brakes 
in  Chap.  vi.  will  make  the  reason  for  this  clear.  The  objection 
to  this  form  of  condenser  is  that  it  takes  up  a  good  deal  of  room, 
and  the  clouds  of  steam  arising  from  it  are  undesirable  in  a  town. 

The  Ejector  Condenser. — In  this  condenser  the  exhaust 
steam  and  the  cooling  water  are  mixed  together,  and  no  air 
pump  is  required.  The  principle  upon  which  the  ejector  con- 
denser works  is  somewhat  similar  to  that  of  the  injector 
previously  described,  but,  instead  of  water  being  fed  into  a 
boiler,  the  cooling  water  and  condensed  steam  are  discharged 
against  atmospheric  pressure  into  a  hot  well.  Unlike  the 
injector,  the  nozzle  through  which  the  steam  passes  is  per- 
forated with  a  large  number  of  openings  through  which  the 
water  comes  in  contact  with  the  exhaust  steam  and  condenses  it. 


174  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

The  ejector  condenser  to  be  really  reliable  should  be  supplied 
with  cooling  water  from  a  tank  20  feet  above  it,  or  under  a 
pressure  of  about  10  Ibs.  per  square  inch  if  supplied  direct  from 
a  centrifugal  pump.  The  advantage  of  this  form  of  condenser  is 
that  it  will  discharge  the  condensed  steam  against  atmospheric 
pressure  while  maintaining  a  vacuum  of  24  or  26  inches  in  the 
engine  without  the  aid  of  an  air  pump. 

The  ejector  form  of  condenser  is  therefore  very  inexpensive, 
and  no  power  is  expended  in  driving  an  air  pump.  Such  a 
condenser  is,  however,  a  little  extravagant  in  cooling  water  ; 
about  40  times  the  weight  of  the  steam  condensed  is  required, 
and  the  same  quantity  of  water  is  required  at  light  as  at  full 
loads.  A  non-return  valve  is  invariably  fitted  between  the 
engine  and  the  condenser,  but,  even  with  this  safeguard,  there  is 
a  certain  element  of  risk — viz.,  that  of  the  water  finding  its  way 
into  the  engine. 

The  danger  can  be  avoided  by  carrying  the  exhaust  pipe  from 
the  engine  about  32  feet  upwards  and  down  again  to  the 
condenser,  as  water  will  not  rise  to  this  height  under  the 
vacuum  formed.  If,  however,  the  pipes  are  carried  up  to  such 
a  height,  a  plain  jet  condenser  of  the  form  shown  by  Fig.  63 
may  be  used.  This  would  then  be  called  a  barometric  condenser. 

Barometric  Condenser. — If  a  condenser,  as  shown  by  Fig. 
63,  is  placed  35  feet  above  the  level  of  the  water  into  which  it 
discharges,  it  will  free  itself  of  water  by  means  of  gravity.  The 
pressure  per 'square  inch  at  the  bottom  of  a  column  of  water  is 
•433  Ib.  for  every  foot  in  height,  so  that  a  column  of  water  34  feet 
high  exerts  a  pressure  of  14-7  Ibs.  per  square  inch.  Even  if  a 
perfect  vacuum  were  formed  in  the  condenser,  the  atmospheric 
pressure  outside  would  not  force  the  water  to  a  height  greater 
than  this.  If,  then,  the  jet  condenser  condenses  the  steam  at, 
say,  35  feet  above  the  level  of  the  discharge,  the  water  will  flow 
away  by  gravity,  and  the  vacuum  will  still  be  maintained. 

It  has  been  found  that  the  drops  of  injection  water  entrain 
any  vapour,  and  that  the  velocity  at  which  the  water  descends 
the  pipe  is  sufficiently  great  to  carry  the  bubbles  down  with  it. 

With  this  form  of  condenser  no  air  pump  is  required,  merely 
a  centrifugal  pump  to  assist  in  raising  the  cooling  water  to  the 
required  height.  The  amount  of  cooling  water  required  is  the 
same  as  in  a  jet  condenser  fixed  in  the  ordinary  way — viz.,  about 
25  times  the  weight  of  the  steam  condensed. 

Cooling  Towers. — The  chief  difficulty  in  connection  with  the 
use  of  condensing  plant  in  large  towns  has  been  the  question  of 
water  supply.  We  have  seen  that  the  water  required  to  con- 
dense the  steam  in  a  surface  condenser  is  40  or  more  times  the 


CONDENSING    PLANT.  175 

weight  of  the  steam  used,  so  that  in  a  large  power  installation 
the  amount  of  cooling  water  required  is  very  great  indeed. 

In  the  case  of  Lancashire  cotton  mills  which  are  not  on  the 
banks  of  a  canal,  it  has  been  customary  to  construct  a  fairly 
large  reservoir  of  water,  known  as  a  lodge,  from  which  the 
cooling  water  is  drawn,  and  to  which  it  is  returned  after  passing 
through  the  condenser.  The  surface  of  the  water  being  exposed 
to  the  atmosphere  gives  up  a  certain  proportion  of  the  heat 
extracted  from  the  steam,  and  although  towards  the  end  of  the 
day  the  temperature  of  the  water,  especially  in  hot  weather, 
becomes  rather  high,  yet  the  system  is  found  to  answer  fairly 
well. 

This  method  of  cooling  the  water,  although  suitable  in  the 
case  of  a  mill  where  the  horse-power  rarely  exceeds  1,000  or 
1,200,  is  not  suitable,  on  account  of  the  large  size  of  reservoir 
required  and  the  expense  of  the  land  needed,  for  a  large  power 
installation  where  many  thousands  of  horse-power  are  developed. 

The  plan  now  usually  adopted  is  to  employ  cooling  towers. 
These  towers,  which  range  from  40  to  80  feet  high,  are  filled 
with  'some  material  suitable  for  breaking  up  a  mass  of  water  and 
exposing  as  much  of  the  surface  as  possible  to  the  atmosphere. 
In  one  make  of  tower  a  large  number  of  short  earthenware 
pipes  are  used ;  they  are  stood  up  end  to  end,  but  the  openings 
of  the  pipes  do  not  come  exactly  over  one  another,  thus  the 
downward  stream  of  water  is  continually  broken  up.  The  water 
from  the  condenser  is  pumped  up  to  the  top  of  the  tower  and 
trickles  down  the  sides  of  the  pipes,  while  a  current  of  air  rises 
up  and  meets  it  and  extracts  a  good  deal  of  heat  from  the  water. 

In  another  make  of  tower,  galvanised  wire  and  timber  slats 
are  used.  Originally  fans  were  used  to  send  a  current  of  air  up 
the  inside  of  the  towers,  but  it  has  been  found  that  in  many 
cases  the  difference  between  the  temperature  of  the  air  inside 
and  outside  the  tower  is  quite  sufficient  to  cause  a  good  draught 
of  air,  and  that  a  fan  can  be  dispensed  with. 

The  tower  stands  over  a  small  tank  or  reservoir  formed  of 
concrete,  into  which  the  water  is  allowed  to  fall  after  passing 
down  the  tower.  In  large  installations  several  towers  are  used. 

A  question  which  may  occur  to  the  student  is — Why  not 
dispense  with  water  altogether  and  use  air  as  the  cooling 
medium  in  the  first  place  1  The  reply  is,  that  it  is  not  possible 
to  do  so,  as  the  specific  heat  of  air  is  too  low,  and  the  volume  of 
air  required  would  be  excessive.  As  nothing  has  yet  been  said 
about  the  specific  of  heat  of  substances,  a  little  digression  must 
be  made. 

Specific  Heat  is  the  amount  of  heat  required  to  raise  1  Ib. 


176  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

of  the  substance  through  1°  F.  Thus  1  British  thermal  unit 
will  raise  1  Ib.  of  water  at  its  greatest  density  through  1°  F. 
The  specific  heat  of  water  is  therefore  said  to  be  1,  and  is 
generally  adopted  as  the  standard  of  comparison. 

The  specific  heat  of  air  is  '238  and  of  cast  iron  '13,  so  that 
the  specific  heat  of  both  air  and  cast  iron  is  less  than  that  of 
water ;  or,  in  other  words,  less  heat  is  required  to  warm  1  Ib.  of 
either  by  1°  than  is  required  to  warm  the  same  weight  (not 
volume)  of  water.  Conversely,  a  gas,  such  as  air,  the  specific 
heat  of  which  is  low,  is  less  capable  of  abstracting  heat  from 
another  substance  or  gas,  with  a  given  rise  of  temperature,  than 
a  body  the  specific  heat  of  which  is  high,  such  as  water.  To 
return  to  the  condensing  question. 

The  specific  heat  of  air  is  -238,  so  that  if  air  were  of  the  same 
weight  as  water,  4 -2  times  more  air  than  water  would  be  required 
for  condensing.  A  cubic  foot  of  air  at  32°  F.,  however,  weighs 
only  -08  Ib.,  while  a  cubic  foot  of  water  weighs  624  Ibs.,  so  that 
water  is  780  times  heavier  than  air.  We  should,  therefore, 
require  a  volume  of  air  780  x  4 '2,  or  3,270  times  greater  than 
that  of  water,  to  obtain  the  same  cooling  effect.  It  would  be 
out  of  the  question  to  deal  with  such  an  enormous  volume  of  air. 

While  upon  the  subject  of  specific  heat,  a  few  words  on  the 
subject  of  the  specific  heat  of  steam  may  be  said,  as  this  is  of 
practical  interest  when  dealing  with  superheated  steam.  The 
specific  heat  of  steam  was  originally  found  by  Regnault  to  be 
about  -48,  and  this  figure  is  usually  taken  to  be  correct,  although 
doubts  have  often  been  expressed  as  to  its  accuracy.  Some 
experiments  recently  made  at  the  Munich  Technical  School 
have  thrown  a  good  deal  of  light  upon  the  subject.  From 
the  curves  reproduced  in  Engineering,  vol.  Ixxxiii.,  p.  227,  it 
would  appear  that  the  specific  heat  of  saturated  steam  varies 
from  -45  at  atmospheric  pressure  to  '62  at  142-24  Ibs.  pressure, 
but  as  soon  as  the  steam  begins  to  get  superheated  the  specific 
heat  falls,  until  at  500°  F.  the  specific  heat  varies  from  -46  to 
•5,  according  to  the  pressure,  while  at  600°  F.  the  specific  heat 
varies  from  '475  to  '498. 


177 


CHAPTER   X. 
THE  STEAM  TURBINE. 

THE  great  success  which  the  steam  turbine  has  achieved  during 
the  last  few  years  seems  to  render  it  probable  that  before 
many  years  are  past  it  will  largely,  if  not  entirely,  supersede 
reciprocating  engines  for  marine  work  and  for  driving  electric 
generators.  Instead  of  huge  engines  having  big  pistons,  with 
heavy  piston-  and  connecting-rods  moving  up  and  down  and 
turning  a  crank,  we  have  in  a  turbine  a  revolving  drum  receiv- 
ing its  motion  direct  from  the  steam. 

The  credit  for  this  transformation  is  almost  entirely  due  to 
the  Hon.  C.  A.  Parsons,  who  first  believed  in  the  possibility  of 
constructing  a  turbine  to  give  considerable  power  without  an 
excessive  consumption  of  steam.  The  first  Parsons  turbine  of 
about  10  H.P.  was  constructed  in  1885,  and  the  first  compara- 
tively large  turbine  in  1 890 ;  and  although  during  the  next 
few  years  a  fair  number  of  turbines  were  made  and  supplied, 
their  introduction  was  a  stiff  uphill  fight.  The  earlier  turbine 
undoubtedly  used  more  steam  than  a  good  reciprocating 
engine,  and  the  makers  of  such  engines  made  the  most  of  the 
fact;  the  turbine  was  referred  to  as  a  steam  eater,  and  many 
were  the  jokes  made  as  to  its  capacity  in  this  respect.  It  was 
not  until  the  Parsons  turbine  was  worked  in  connection  with  a 
condenser  that  it  was  able  to  compete  on  terms  of  equality,  as 
regards  consumption  of  steam,  with  a  reciprocating  engine. 
When  the  figures  obtained  at  the  first  authentic  condensing 
turbine  trial  were  published  they  were  received  with  a  certain 
amount  of  incredulity,  but  from  that  date  the  advance  of  the 
turbine  into  general  favour  has  been  steady  and  continuous. 

The  Parsons  turbine  is  of  the  parallel  flow,  reaction  *  type — 
i.e.,  one  in  which  the  steam  flows  through  the  turbine  in  a 
direction  parallel  with  its  axis,  as  shown  by  Fig.  69.  The 
illustration  shows  a  Parsons  -  Willans  turbine  (the  difference 
between  this  and  the  original  Parsons  turbine  is  explained 
later)  without  bearings  or  governor.  The  blades  in  Fig.  69 
are  shown  diagrammatically — i.e.,  a  row  of  blades  is  represented 
by  one  line ;  they  are  shown  in  detail  by  Figs.  70  and  72.  The 

*  The  meaning  of  the  term  reaction  turbine  is  explained  in  the  Chapter 
on  Water  Turbines. 

12 


178 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


rotating  drum  or  rotor  is  provided  with  a  large  number  of 
blades,  and  between  each  row  of  moving  blades  there  is  a  row 
of  fixed  guide  blades  attached  to  the  casing,  as  shown  by  Fig.  70. 

Steam  at  high  pressure  is  admitted  at  A,  Fig.  69.  It  passes 
through  the  guide  blades  and  impinges  on  a  row  of  blades 
attached  to  the  drum;  it  exerts  a  reactionary  force  on  these 
blades,  causes  them  to  move  and  thus  rotates  the  drum  to  which 
they  are  attached.  After  passing  through  the  next  row  of  guide 
blades  the  steam  impinges  on  the  succeeding  row  of  blades,  and 
so  on,  until  the  steam  is  fully  expanded,  when  it  passes  away  to 
the  condenser. 

It  will  be  seen  from  Fig.  69  that  at  first  the  blades  are  short, 
they  are  also  closely  spaced ;  as  the  steam  expands  blades  of  a 
greater  length  and  width,  and  more  coarsely  pitched,  are  used. 

Steam 
Inlet 


Dummy 
Pistons 


Outlet 


Fig.  69. — Willans-Parsons  turbine. 


The  drum  also  is  increased  in  diameter ;  this  allows  of  a  larger 
number  of  blades  being  used,  and  the  somewhat  weaker  turning 
effort  of  the  steam  on  them  is  made  up  for  by  their  increased 
distance  from  the  centre  of  the  drum,  or,  in  other  words,  by  their 
increased  speed.  By  the  time  the  steam  has  reached  the  end  of 
the  turbine  it  has,  by  a  long  series  of  steps,  fallen  in  pressure 
and  has  imparted  a  large  portion  of  its  energy  to  the  rotating 
drum. 

In  order  to  prevent  end  thrust  the  drum  is  enlarged  at  the 
high-pressure  end  of  the  turbine ;  these  enlargements  are  called 
dummy  pistons,  and  are  provided  with  baffle  rings,  as  shown  by 
Fig.  71.  The  turbine  is  usually  arranged  so  that  high-pressure 
steam  can  be  admitted  by  a  pipe  or  passage  to  the  low-pressure 
end;  this  enables  additional  power  to  be  obtained  for  short 
periods,  but  of  course  uneconomically.  The  pipes  shown  under- 


THE    STEAM    TURBINE. 


179 


neath  the  turbine  are  for  balancing  purposes.  The  small  pipe 
allows  steam  at  a  pressure  corresponding  with  that  in  the  middle 
of  the  turbine  to  press  against  the  face  of  the  dummy  piston, 


Fig.  70. 


Fig.  72. 


S=Shrouding 
F=  Foundation 

Ring. 
C=  Caulking 

Strip 


Baff/e  Kings 

round 
Dummy  Pisbons. 

Fig.  71. 
Figs.  70  to  72.— Turbine  blading. 

while  the  large  pipe  places  the  back  of  the  piston  in  communi- 
cation with  the  condenser. 

In  the  Parsons  turbine  the  governor  is  driven  from  the  main 
shaft  by  a  worm  and  wheel,  and  is  arranged  so  that  the  steam  is 


180  MECHANICAL    ENGINEERING   FOR    BEGINNERS. 

admitted  intermittently  or  in  puffs  ;  when  the  turbine  is  lightly 
loaded  the  interval  between  the  puffs  is  longer  than  when  fully 
loaded.  With  a  full  load  on  the  turbine  the  puffs  are  almost 
continuous.  The  effect  of  this  arrangement  is  that  the  turbine 
is  always  supplied  with  steam  at  high  pressure  even  when 
working  at  light  loads.  If  the  governor  worked  on  the  ordinary 
throttling  system  the  turbine,  at  light  loads,  would  be  supplied 
with  steam  at  a  pressure  considerably  less  than  that  of  the 
boiler. 

The  consumption  of  steam  in  a  Parsons'  3,500  kilowatt 
turbine,  say  5,100  brake  horse-power,  working  with  200  Ibs. 
steam  pressure,  and  121°  of  superheat,  has  been  as  low  as  13 '2 
Ibs.  per  kilowatt  hour.  This  is  equivalent  to  9 '85  Ibs.  per 
electrical  horse-power,  and  about  9  Ibs.  per  brake  horse-power. 
The  best  result  obtained,  within  the  author's  knowledge,  with  a 
triple-expansion  reciprocating  engine  working  with  the  same 
pressure  and  the  same  degree  of  superheat,  has  been  11 '9  Ibs. 
per  brake  horse-power. 

The  Willans- Par  sons  Turbine. — The  principle  upon  which 
this  turbine  works  is  practically  the  same  as  the  Parsons,  but 
there  are  certain  differences  in  the  construction.  In  the  Parsons 
turbine  the  blades  are  placed  separately  in  grooves  cut  in  the 
rotor  and  casing  and  are  held  in  position  by  small  pieces  of 
bronze  which  are  wedged  or  caulked  in  between  the  blades,  the 
ends  of  the  blades  being  free.  In  the  Willans-Parsons  turbine 
the  outer  ends  of  the  blades  are  riveted  into  an  encircling 
U-shaped  ring  of  bronze,  as  shown  by  Fig.  70.  The  root  of 
each  blade  is  inserted  into  a  saw  cut  in  a  ring,  one  side  of  the 
blade  at  the  root  is  turned  over,  and  the  whole  is  wedged  tightly 
in  its  groove  by  a  caulking  strip.  The  blades  with  their  rings 
are  put  into  position  in  sections. 

The  blades  shown  by  the  illustration  are  drawn  to  approxi- 
mately half  their  actual  size,  and  represent  those  used  about 
half  way  along  the  rotor  of  a  1,500  kilowatt  (2,200  B.H.P.) 
turbine  running  at  1,500  revs,  per  minute.  In  such  a  turbine 
the  blades  range  in  length  from  1  or  1J  inches  at  the  high- 
pressure  end  to  about  6  inches  at  the  low-pressure  end.  In  large 
marine  turbines  running  at  a  considerably  lower  speed  the  blades 
are  much  longer  and  wider.  The  blades  at  the  low-pressure  end 
of  one  of  the  turbines  for  an  Atlantic  liner,  the  "  Mauretania," 
are  about  23  inches  long  and  2  inches  wide. 

The  U-shaped  shrouding  ring  at  the  ends  of  the  blades  is 
used  chiefly  in  the  Willans-Parsons  turbine  (and  by  Messrs. 
Yarrow) ;  in  the  original  Parsons  turbine  the  blades  were 
strengthened  by  a  ring  let  into  the  blades,  and  soldered  to  them 


THE    STEAM    TURBINE. 


181 


by  silver  solder,  as  shown  by  Fig.  72,  at  the  top  right-hand 
corner  of  the  illustration ;  if  the  blades  were  very  long,  two  or 
more  rings  were  used. 

The  shrouding,  besides  strengthening  the  blades,  has  two  other 
advantages — viz.,  should  any  whipping  of  the  shaft  or  hogging 
of  the  casing  take  place  and  cause  the  blades  to  touch  the  casing, 
the  shrouding  will  merely  rub  against  the  latter ;  the  shrouding 
will  certainly  wear  away,  but  the  blades  will  not  be  stripped 
away,  as  is  the  case  if  the  ends  are  unprotected.  The  other 
advantage  of  shrouding  the  ends  and  having  a  ring  round  the 
roots  of  the  blades,  is  that  the  flow  of  steam  past  the  ends  is  to  a 
certain  extent  checked,  and  the  loss  due  to  leakage  past  the  ends 
is  probably  less  than  with  blades  having  free  ends. 

The  consumption  of  steam  is  approximately  the  same  as  in  the 
Parsons  turbine. 

In  some  of  the  Willans-Parsons  turbines  the  upper  portion  of 
the  casing  is  hinged,  so  that  there  is  no  danger  of  damaging  the 
blading  when  the  upper  half  is  being  opened  up  for  inspection  or 
during  replacement.  All  pipes  and  connections  are  attached 
to  the  lower  half  of  the  casing.  The  Willans-Parsons  turbine 
is  governed  by  throttling,  and  not  by  admitting  steam  inter- 
mittently. 

The  Brush-Parsons  Turbine. — This  turbine  is  very  similar 
to  the  Parsons ;  it  differs  only  in  minor  points  of  construction ; 
for  instance,  the  blades  are  constructed  with  a  strengthening  ring 
let  into  their  ends,  instead  of  into  one  side  as  in  the  Parsons.  A 
special  centrifugal  water  gland  is  used  for  packing  the  shaft  at 
the  ends  of  the  turbine. 

The  speeds  at  which  the  Parsons,  Willans-Parsons,  and  Brush  - 
Parsons  turbines  are  usually  designed  to  run,  when  driving 
electric  generators,  are  as  follows  : — 

TABLE   XVII. 


Electrical  Output. 

Brake  H.P. 

Revolutions  per  Minute. 

500  Kilowatts. 

730 

1,500  to  3,000 

750 

1,100 

1,500  „  3,000 

1,000 

1,450 

1,500  ,,  1,800 

1,500 

2,200 

1,500  „  1,800 

3,000 

4,400 

750  „  1,500 

5,000 

7,300 

750  ,,  1,000 

For  marine  work  the  speed  of  the  turbine  is  much  less ;  for 
instance,    the   H.P.    and   L.P.   turbines   in   the   Cunard   Liner 


182  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

"  Lusitania,"  which  collectively  give  about  64,000  H.P.,  run  at 
about  188  revs,  per  minute. 

When  the  turbine  is  coupled  direct  to  an  alternator,  the  exact 
speed  is  determined  by  the  periodicity  of  the  latter.  The  meaning 
of  the  periodicity  of  an  alternator  is  given  in  the  electrical 
chapter.  The  peripheral  speed  of  the  blades  in  turbines  of  the 
Parsons  type  does  not  usually  exceed  300  feet  per  second. 

The  De  Laval  Steam  Turbine. — This  turbine,  which  was 
invented  by  a  Swedish  engineer,  works  on  a  principle  different 
from  that  of  the  Parsons.  In  the  latter  the  steam  is,  as  we  have 
seen,  expanded  from  its  highest  to  its  lowest  pressure  through  a 
long  succession  of  steps,  each  row  of  blades  absorbing  a  small 
part  of  the  energy  of  the  steam. 

In  the  De  Laval  turbine,  the  steam  is  expanded  in  one  step 
from  its  highest  to  its  lowest  pressure.  This  expansion  is  carried 
out  in  a  nozzle.  It  has  been  proved  by  experiment  that,  although 
steam  loses  its  pressure  if  expanded  from  a  small  volume  to  a 
larger  volume,  it  does  not  lose  its  temperature  or  energy,  pro- 
vided it  does  no  external  work  during  such  expansion.* 

In  the  De  Laval  turbine  this  fact  is  made  use  of,  so  that, 
instead  of  having  a  small  volume  of  steam  issuing  from  a  nozzle 
at  a  high  pressure  (in  which  case  much  of  the  steam  would 
expand  in  the  air  after  striking  the  buckets,  and  its  energy  be 
lost),  the  nozzles  are  constructed  so  that  the  steam  is  expanded 
in  them  before  leaving  the  orifice.  We  have,  therefore,  a  very 
large  volume  of  steam  at  low  pressure  travelling  at  high  speed ; 
by  allowing  the  steam  to  impinge  on  buckets,  also  travelling  at  a 
high  speed,  we  utilise  a  very  large  part  of  its  energy.  A  turbine 
working  on  this  principle  is  called  an  impulse  turbine.  The 
meaning  of  the  term  impulse  turbine  is  explained  in  the  chapter 
dealing  with  water  turbines. 

Fig.  73  shows  the  ring  of  buckets  and  four  nozzles  of  the  De 
Laval  turbine;  the  illustration  shows  clearly  the  action  of  the 
steam  on  the  blades.  In  order  to  take  the  greatest  advantage 
of  the  energy  of  the  steam,  the  ring  of  buckets  must  travel  at 
a  very  high  speed  indeed ;  the  peripheral  speed  of  the  buckets 
should  be  47  per  cent,  of  the  velocity  of  the  steam,  so  that,  if  the 
steam  leaves  the  nozzle  at  a  speed  of  4,000  feet  per  second,  the 

*  Joule's  law  is — "When  a  gas  expands  without  doing  external  work, 
and  without  taking  in  or  giving  out  heat,  its  temperature  does  not  change." 
To  prove  this  rule,  Joule  connected  a  vessel  containing  compressed  gas 
with  another  vessel  that  was  empty  by  means  of  a  pipe  with  a  closed  stop 
cock.  Both  vessels  were  immersed  in  a  tub  of  water,  and  were  allowed  to 
assume  a  uniform  temperature.  Then  the  stop  cock  was  opened,  the  gas 
expanded  without  doing  external  work,  and  finally  the  temperature  of  the 
water  in  the  tub  was  found  to  have  undergone  no  change. 


THE    STEAM    TURBINE. 


183 


peripheral  speed  of  the  buckets  should  be  1,880  feet  per  second; 
but  for  practical  reasons  the  speed  is  considerably  less. 

In  the  case  of  a  300  H.P.  turbine  the  outside  diameter  of  the 
wheel  is  31 J  inches;  it  runs  at  10,600  revs,  per  minute,  giving 
a  peripheral  speed  of  about  1,457  feet  per  second  or  87,420  feet 
per  minute.  With  smaller  turbines  the  peripheral  speed  is  less. 


Fig.  73. — Bucket  wheel  of  De  Laval  turbine. 

To  enable  a  wheel  to  run  at  this  enormous  speed  several 
interesting  methods  of  construction  have  been  adopted.  The 
wheel  is  of  the  disc  form,  and  in  the  larger  turbines  it  is  solid 
throughout ;  there  is  not  even  a  hole  through  the  boss,  the  shaft 
being  bolted  to  the  wheel  by  flanges  on  each  side;  the  blades 
or  buckets  are  dovetailed  into  the  rim.  The  centrifugal  force 
on  these  buckets  is  considerable;  a  bucket  weighing  \  oz. 


184 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


if  rotated  at  a  radius  of  1*25  feet,  will  develop  a  centrifugal 
force  of  about  134  cwts.,  when  run  at  a  speed  of  10,600  revs, 
per  minute.  The  weight  of  a  bucket  of  a  300  H.P.  turbine  is  a 
little  heavier  than  \  oz.  The  ends  of  the  buckets  form  what  is 
practically  an  encircling  ring  ;  it  would  be  impossible  to  surround 
the  buckets  (of  the  larger  turbines)  with  an  encircling  ring  of 
steel,  as  is  sometimes  stated  to  be  the  case.  If  the  reader  will 
work  out  the  stress  in  such  a  ring  due  to  centrifugal  force 
(the  rule  is  given  in  Chapter  vn.),  he  will  find  that  it  amounts 
to  about  95  tons  per  square  inch,  or  sufficient  to  burst  any  steel 
ring.  Each  bucket  is  firmly  dovetailed  into  the  disc,  and  has 
only  to  withstand  the  stress  due  to  its  own  centrifugal  force. 


Fig.  74. — De  Laval  turbine  with  gearing. 

The  shaft  carrying  the  bucket  wheel  is  of  small  size ;  that  of  a 
300  B.H.P.  turbine  is  only  1T5F  inches  diameter,  the  reason  for 
this  being  that  it  is  impossible  to  insure  that  the  centre  of  the 
shaft  shall  be  absolutely  in  the  centre  of  gravity  of  the  wheel,  or, 
in  other  words,  that  the  wheel  shall  be  in  perfect  balance.  If 
a  very  stiff  shaft  were  used  the  effect  of  a  slight  want  of  balance 
would,  at  the  very  high  speeds  employed,  be  to  cause  excessive 
vibration.  By  employing  a  light  and,  therefore,  somewhat 
flexible  shaft,  the  shaft  is  enabled  to  spring  somewhat ;  when 
the  wheel  reaches  about  \  or  \  of  its  full  speed,  it  settles  itself 
on  a  new  centre  so  as  to  run  smoothly  and  free  from  vibration. 

The  speed  of  the  De  Laval  turbine,  which  ranges  from  30,000 
revs,  per  minute  in  the  5  H.P.  turbine,  to  10,600  revs,  in  the 


THE    STEAM    TURBINE.  185 

300  B.H.P.  turbine,  is  too  high  to  enable  it  to  drive  even  a 
dynamo  if  directly  coupled  to  it.  The  speed  is  therefore  reduced 
by  double  helical  gearing,  as  shown  by  Fig.  74.  The  two  halves 
of  the  helical  teeth  are  slightly  separated,  so  that  they  can  be 
machine  cut.  The  pinion  is  made  in  one  piece  with  the  shaft, 
and  the  linear  velocity  of  the  teeth  is  about  1,000  feet  per  second. 
The  helical  wheels  are  enclosed  in  a  casing,  as  shown  by  dotted 
lines.  The  illustration  shows  the  turbine  driving  a  belt  pulley, 
but  a  dynamo,  pump,  or  fan,  can  be  placed  in  the  position 
occupied  by  the  pulley. 

The  turbine  is  provided  with  several  nozzles,  some  of  which 
are  usually  closed  when  working  with  high  steam  pressures  and 
condensing;  some  of  the  nozzles  are  also  closed  when  the  turbine 
is  working  lightly  loaded. 

A  consumption  of  18 '9  Ibs.  per  kilowatt  hour,  working  with 
193  Ibs.  pressure,  and  with  60°  of  superheat,  has  been  recorded. 
The  De  Laval  turbine  is  made  in  this  country  by  Messrs. 
Greenwood  &  Batley  of  Leeds. 

The  Curtis  turbine,  made  by  the  British  Thomson-Houston 
Co.,  is  of  the  "  Impulse"  type,  but  is  designed  so  that  an 
extremely  high  rotative  speed  is  not  necessary  in  order  to 
obtain  good  results. 

In  the  Curtis  turbine,  as  in  the  Laval,  the  steam  is  expanded 
in  nozzles  before  striking  the  buckets,  but  not  to  the  same 
extent.  The  nozzles  are  designed  so  that  the  speed  of  the 
steam  issuing  from  them  shall  be  about  2,000  feet  per  second. 
After  the  steam  has  passed  through  one  row  of  buckets  its 
course  is  altered  by  stationary  buckets  or  blades,  so  that  the 
steam  shall  impinge  to  the  best  advantage  on  a  second  row  of 
moving  buckets.  After  the  steam  has  passed  through  this  second 
row  of  buckets,  which  completes  one  stage,  it  is  again  passed 
through  nozzles,  and  goes  through  a  course  similar  to  that  just 
described — viz.,  moving  buckets,  stationary  buckets,  and  moving 
buckets. 

Fig.  75*  shows  the  arrangement  of  nozzles  and  buckets  in  a 
two  stage  turbine.  The  two  rows  of  moving  buckets  are  bolted 
to  the  upper  and  lower  sides  of  one  rotating  disc. 

Fig.  76  shows  the  general  arrangement  of  the  Curtis  turbine 
driving  a  dynamo,  the  latter  being  placed  immediately  above 
the  turbine. 

It  will  be  seen  from  Fig.  76  that  the  Curtis  turbine,  in  its 
larger  sizes,  has,  unlike  the  Parsons  and  the  De  Laval,  a  vertical 

*  Fig.  75  causes  a  curious  optical  illusion,  each  row  of  buckets  apparently 
varying  in  width.  Measurement  by  means  of  a  pair  of  dividers  will  show 
that  this  is  not  so. 


186  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

shaft.  This  shaft,  which  carries  the  whole  of  the  revolving 
portions  of  the  turbine  and  dynamo,  is  supported  by  a  footstep 
bearing.  The  latter  is  made  in  halves;  the  surfaces  are  kept 
apart  by  a  film  of  oil  or  water  supplied  under  pressure.  The 
lubricant,  after  leaving  the  footstep  bearing,  passes  upwards,  and 
lubricates  a  guide  bearing  which  keeps  the  shaft  central.  The 
upper  guide  bearings  at  A  and  B  are  also  lubricated  by  oil 
supplied  under  pressure ;  after  use,  the  oil  passes  to  a  tank,  and 
is  used  over  and  over  again. 

The  governing  of  the  Curtis  turbine  is  effected  by  opening 
or  closing  the  valves  controlling  the  nozzles,  as  shown  by 
Fig.  75,  the  amount  of  steam  admitted  to  the  turbine  being 
proportional  to  the  load ;  the  steam  pressure  is  not  reduced  by 
throttling. 

STEAM  CHEST 


NOZZLE 

MOVING  BLADES 
6TATFONARY  BLADES 
MOVING  BLADES 


NOZZLE 

DIAPHRAGM 

MOVING  BLADES 
STATIONARY  BLADES 
MOVING  BLADES 


1      !     I       I       I       I 

Fig.  75. — Curtis  turbine  blading. 

The  buckets  of  the  Curtis  turbine  are  cut  out  of  a  solid  ring 
which  is  bolted  to  a  disc.  A  thin  encircling  ring  of  steel 
surrounds  the  outer  ends  of  the  buckets. 

The  clearances  of  the  Curtis  turbine,  unlike  those  of  the 
Parsons,  can  be  adjusted  to  a  nicety  by  means  of  a  strong  screw 
at  the  bottom  of  the  footstep  bearing.  In  the  larger  turbines 
this  screw  is  worked  by  a  worm  and  wheel.  The  buckets  being 
shrouded,  if  any  rubbing  should  occur  the  shrouding  will  wear, 
but  the  buckets  will  not  be  destroyed. 

The  lower  part  of  the  Curtis  turbine  usually  contains  the 
surface  condenser.  The  Curtis  turbine  occupies  much  less  floor 
space  than  one  of  the  Parsons  type,  and  the  construction  of  the 


THE    STEAM    TURBINE. 


187 


Fig  76.— Curtis  turbine. 


188  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

buckets,  cut  out  of  the  solid  as  they  are  and  shrouded,  is  a  good 
feature. 

The  consumption  of  steam  in  Curtis  turbines  usually  employed 
for  driving  generators  up  to  3,000  kilowatts,  working  with  super- 
heated steam  and  with  a  good  vacuum,  is  frequently  about  16 '5 
Ibs.  per  kilowatt  hour ;  but  in  the  case  of  a  10,000  kilowatt  five- 
stage  turbine,  running  at  750  revs,  per  minute,  a  consumption  as 
low  as  12-9  Ibs.  per  kilowatt  has  been  recorded.  The  steam 
pressure  was  176  Ibs.;  superheat,  147°;  vacuum,  29*47  inches. 
This  turbine  was  one  of  four  erected  at  the  Fisk  Generating 
Station,  Chicago. 

The  Rateau  Turbine. — The  Rateau  turbine,  made  by  Messrs. 
Fraser  <fe  Chalmers,  is  similar  in  principle  to  the  Curtis,  but  the 
shaft  is  horizontal  instead  of  vertical.  This  turbine  has  been 
employed  to  a  considerable  extent  on  the  Continent,  fre- 
quently being  driven  by  the  exhaust  steam  from  reciprocating 
engines. 

In  cases  where  the  steam  is  only  supplied  intermittently,  a 
heat  accumulator  is  used.  This,  in  its  simplest  form,  consists  of 
an  old  boiler  shell  filled  with  scrap  iron,  into  which  the  exhaust 
steam  is  taken,  and  from  which  the  turbine  draws  its  supply. 
The  exhaust  steam,  on  entering  the  shell,  gives  up  some  of  its 
heat  to  the  scrap  iron,  and  partially  condenses  ;  when  the  supply 
of  exhaust  steam  from  the  reciprocating  engine  ceases,  and  the 
turbine  goes  on  drawing  its  steam,  the  pressure  in  the  shell  falls, 
and  the  water  lying  in  it  turns  into  steam,  and  so  keeps  up  the 
supply.  The  turbine  must  work  condensing;  otherwise,  steam  at 
the  somewhat  low  pressure  available  would  be  of  comparatively 
little  use.  A  more  elaborate  heat  accumulator  consists  of  a  shell 
containing  shallow  trays  of  water,  or  of  a  drum  partly  filled  with 
water,  and  containing  tubes  through  which  the  steam  passes,  the 
tubes  being  arranged  so  that  a  good  circulation  of  water  is  main- 
tained around  them,  but  the  principle  is  the  same. 

Westinghouse  Turbine. — In  this  turbine  the  impulse 
principle  is  combined  with  the  reaction  or  Parsons  system. 
Steam  is  admitted  at  the  centre  of  the  turbine,  and  flows 
outwards  towards  the  ends.  It  impinges  first  on  two  impulse 
wheels,  as  in  the  De  Laval  turbine ;  after  passing  the  impulse 
wheels  the  steam,  which  has  dropped  to  about  one-third  of  its 
original  pressure,  passes  through  blading  of  the  Parsons  type, 
and  leaves  at  each  end  of  the  turbine.  By  this  arrangement 
dummy  balancing  pistons  are  not  required,  and  the  overall  length 
of  the  turbine  is  reduced. 

A  somewhat  similar  turbine  has  recently  been  constructed  by 
Messrs.  Melms,  Pfenninger  &  Sankey,  but  in  it  the  steam  is 


THE    STEAM    TURBINE.  189 

admitted  at  one  end,  strikes  an  impulse  wheel,  and  then  passes 
through  blading  of  the  Parsons  type  to  the  other  end,  where  it 
goes  to  the  condenser.  With  a  500-kilowatt  turbine  constructed 
on  this  principle,  a  consumption  of  17  "2  Ibs.  per  kilowatt  hour 
has  been  recorded. 

The  Zoelly  Turbine  works  on  the  same  principle  as  the 
Curtis ;  but,  like  the  Rateau,  the  shaft  is  horizontal.  There  are 
usually  ten  rotary  wheels  having  buckets  round  the  periphery ; 
these  wheels  are  placed  in  two  separate  casings ;  the  shaft  runs 
right  through  the  casings,  and  is  supported  by  a  bearing  placed 
between  them.  One  casing  is  the  high-pressure  portion  of  the 
turbine,  the  other  the  low-pressure  portion.  The  steam,  after 
passing  through  the  high-pressure  portion,  is  conveyed  by  a  pipe 
to  the  low-pressure  portion.  If  the  reader  will  turn  the  illustra- 
tion of  the  Curtis  turbine  up  sideways,  so  that  the  shaft  is 
horizontal,  and  imagine  that  the  dynamo  shown  is  the  high- 
pressure  portion  of  the  turbine,  he  will  have  a  good  idea  as  to 
the  general  appearance  of  the  Zoelly  turbine. 

In  this  turbine  the  blades  are  made  of  nickel  steel,  and  the 
section  decreases  from  the  roots  to  the  tips. 

General  Remarks  on  the  Steam  Turbine, — It  has  already 
been  said  that  turbines  of  the  Parsons  type  when  coupled  to 
dynamos  have  given  results,  as  regards  steam  consumption, 
which  have  not  been  equalled  by  reciprocating  engines.  It 
may,  therefore,  be  well  to  look  for  the  reasons  which  enable 
such  economy  to  be  obtained.  In  the  first  place  there  is  but 
little  initial  condensation,  as  the  steam  after  doing  its  work 
passes  away  at  the  end  of  the  turbine  farthest  from  that  at 
which  it  was  admitted.  Condensation  in  the  turbine  can  also 
be  reduced  by  superheating  the  steam  to  a  high  degree,  and  it 
must  be  remembered  that  a  turbine  can  be  supplied  with  super- 
heated steam  with  much  less  risk  of  injury  than  a  reciprocating 
engine,  as  in  the  former  there  are  no  rubbing  surfaces  in  contact 
with  the  steam.  The  bearings  require  to  be  oiled,  but  these 
are  outside  the  turbine  proper,  and  the  steam  does  not  reach 
them. 

Another  point  which  conduces  to  economy  is  this — owing 
to  the  absence  of  internal  friction,  it  is  worth  while  to  expand 
the  steam  to  a  point  much  further  than  would  be  useful  in 
the  case  of  a  reciprocating  engine.  In  a  turbine,  too,  it  is 
possible  to  provide  a  very  large  opening  through  which  the 
steam  can  pass  away  to  the  condenser,  as  the  reader  will  see  by 
referring  to  Fig.  69.  It  is  found  that  a  difference  of  even  half 
an  inch  in  the  vacuum  makes  an  appreciable  difference  in  the 
consumption  of  steam.  The  reader  will  have  seen  from  the 


190  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

table  given  in  the  last  chapter  that  the  volume  of  steam 
increases  very  rapidly  as  the  vacuum  approaches  a  perfect  one; 
in  fact,  the  volume  of  steam  with  a  29-inch  vacuum  is  double 
that  of  steam  with  a  26-inch  vacuum,  so  that  with  the  high 
vacuum  we  have  double  the  volume  of  steam  acting  on  the  last 
blades  of  the  turbine.  The  saving  of  steam  due  to  the  last  few 
inches  of  vacuum  is  at  the  rate  of  between  4  and  5  per  cent,  per 
inch  of  vacuum. 

The  economy  of  a  steam  turbine  would  probably  be  still 
greater  were  it  not  for  the  necessity  of  having  clearance 
between  the  ends  of  the  moving  blades  and  the  casing,  and 
between  the  ends  of  the  fixed  blades  and  the  drum.  Owing 
to  the  somewhat  great  length  of  the  turbine,  the  casing  has  a 
tendency  to  "  hog  "  under  the  temperature  and  pressure  of  the 
steam.  The  drum,  too,  has  a  tendency  to  "whip"  owing  to  its 
length  and  high  speed,  and  to  the  impossibility  of  making  a 
perfect  balance  of  the  blades.  To  insure  that  the  blades  will 
not  come  in  contact  with  the  casing,  it  is  necessary  to  give  them 
a  certain  clearance,  and  this  clearance  allows  a  percentage  of 
steam  to  pass  without  doing  any  effective  work.  In  small 
turbines  the  clearance  is  proportionately  greater  than  in 
turbines  of  large  power,  and  it  is  due  to  this  fact  that 
turbines  of  less  than  1,000  H.P.  cannot  at  present  compete 
successfully  on  the  score  of  economy  with  triple-expansion 
reciprocating  engines. 

In  spite  of  this  clearance  loss,  the  fact  remains  that  turbines 
of  large  size  (as  already  stated)  are  more  economical  than 
reciprocating  engines.  Apart  from  the  economy  attained  with 
turbo-electric  generating  plant,  it  has  been  found  that  the  coal 
used  on  the  Channel  steamers  fitted  with  turbines  is  from 
15  to  20  per  cent,  less  than  on  those  fitted  with  reciprocating 
engines. 

Other  points  in  favour  of  the  turbine  for  marine  work  are 
these  : — The  weight  of  turbines  and  boilers  for  propelling  a  ship 
is  about  5  per  cent,  less  than  that  of  reciprocating  engines  and 
boilers  of  the  same  power.  In  small  vessels  of  the  torpedo- 
destroyer  class,  the  whole  of  the  turbine  can  be  placed  below 
the  water  line,  so  that  there  is  less  risk  of  damage  from  gun  fire 
than  is  the  case  with  a  reciprocating  engine.  For  passenger 
steamers  the  absence  of  vibration  is  a  great  advantage.  A  good 
feature  of  the  turbine  for  both  land  and  marine  work  is  the 
fact  that  no  oil  is  required  in  the  turbine  itself ;  the  condensed 
steam  may  therefore  be  pumped  back  into  the  boiler  without  the 
intervention  of  oil  filters. 

Both  for  land  and  marine  work  the  turbine  has  proved  itself 


THE    STEAM    TURBINE.  191 

to  be  thoroughly  reliable.  An  incident  which  was  related  to  the 
author  a  good  many  years  ago  by  the  engineer  concerned  may 
not  be  considered  out  of  place.  Amongst  the  earlier  turbines 
made  by  Messrs.  Parsons  were  some  constructed  for  one  of  the 
London  electric  light  companies.  These  turbines  were  placed 
on  the  first  floor  of  the  electric  generating  station,  while  the 
ground  floor  was  occupied  by  high-speed  reciprocating  engines. 
When  a  serious  fire  broke  out  one  night,  the  greatest  efforts 
were  made  to  keep  up  a  supply  of  current,  and  the  engineer 
succeeded  in  keeping  the  turbine  sets  running  after  all  the 
reciprocating  sets  had  failed  owing  to  the  heat  and  water. 
The  load  which  the  turbine  sets  had  to  take  up  was  enormous, 
but  by  throwing  buckets -of  water  on  the  dynamos  to  keep  them 
cool,  the  turbine  sets  came  through  the  ordeal  triumphantly. 

The  reliability  of  the  turbine,  together  with  the  fact  that  it 
may  be  over-loaded  without  serious  risk  and  without  a  great 
falling  off  in  economy,  are  points  very  greatly  in  its  favour. 
In  the  case  of  electric  generating  stations,  the  saving  effected 
in  the  cost  of  dynamos,  and  of  buildings  and  foundations,  to  say 
nothing  of  the  saving  in  oil  and  engine-room  attendants,  is  very 
marked  indeed. 

The  limitations  of  the  turbine  are  chiefly  those  due  to  its  high 
speed ;  the  speed  is  too  high  to  enable  it  to  drive  by  belts  or 
ropes  without  the  intervention  of  gearing.  If,  however,  the 
electrical  method  of  transmitting  power  is  adopted  this  objection 
d/aes  not  hold  good.  One  rolling  mill  has  recently  adopted  this 
method  of  transmission ;  in  this  case  steam  turbines  are  used 
for  driving  dynamos,  and  the  rolls  for  rolling  steel  rails  are 
driven  by  motors.  The  arrangement  is  said  to  give  entire  satis- 
faction. A  similar  arrangement  has  recently  been  introduced 
into  a  cotton  mill. 

The  steam  turbine  is  somewhat  handicapped  by  the  fact  that 
it  does  not  reverse.  On  board  ship  this  difficulty  is  overcome  by 
having  separate  turbines  for  reversing.  In  marine  work  several 
turbines  are  usually  employed  for  driving  the  ship ;  for  instance 
in  the  "  Carmania,"  an  Atlantic  liner,  one  high-pressure  turbine 
drives  a  propeller  amidships  and  two  low-pressure  turbines  drive 
the  port  and  starboard  propellers.  The  reversing  turbines  also 
drive  these  shafts ;  when  going  ahead  the  reversing  turbines 
remain  connected  to  the  condenser,  steam  being,  of  course,  shut 
off. 

Another  fact  which  has  somewhat  retarded  the  introduction 
of  the  turbine  is  that  it  must  condense  if  it  is  to  complete 
successfully  with  a  reciprocating  engine.  In  large  towns  water 
must  be  paid  for  at  a  fairly  high  rate  and  this  charge  would  be 


192  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

prohibitive  if  the  cooling  water  were  used  only  once  and  then 
thrown  away ;  but  the  tendency  now  is  for  generating  stations 
to  be  built  on  the  outskirts  of  large  towns  where  sufficient  land 
is  available  to  admit  of  cooling  towers  being  erected.  The  water 
is  cooled  in  these  towers  and  is  used  over  and  over  again. 

On  board  ship  the  difficulty  does  not  occur  as  sea  water  is 
available  for  cooling  purposes. 


193 


CHAPTER    XI. 
ELECTRICAL    CHAPTER. 

IT  is  almost  essential  at  the  present  time  that  an  engineer 
should  have  some  knowledge  of  electrical  matters,  and  while  no 
attempt  will  be  made  to  go  at  all  deeply  into  the  subject  in  the 
present  book,  yet  a  few  words  dealing  with  the  main  facts  of 
practical  work  and  with  the  relation  of  electrical  units,  such  as 
the  volt,  ampere,  and  ohm,  to  mechanical  units  of  work,  such  as 
horse-power,  may  be  useful. 

Production  of  the  Electric  Current. — In  all  cases  where 
current  is  required  in  any  quantity  or  at  any  useful  pressure 
(electromotive  force)  it  is  produced  by  a  dynamo  or  alternator, 
and  the  principle  upon  which  these  machines  work  will  now  be 
described. 

In  an  ordinary  horse-shoe  magnet,  such  as  may  be  purchased 
in  a  toy  shop,  there  are  two  poles,  one  north,  the  other  south ;  if 
the  magnet  is  straightened  out  as  in  a  mariner's  compass  the 
north  pole  (or  north-seeking  pole)  will  point  to  the  north  and 
the  south  pole  to  the  south.  Surrounding  each  pole  is  a 
magnetic  field,  and  for  the  purpose  of  making  calculations  in 
designing  dynamos  the  strength  or  weakness  of  a  magnetic  field 
is  expressed  by  a  number  of  imaginary  magnetic  lines  of  force 
per  square  inch  of  the  magnet  face.  When  the  two  poles  of  a 
magnet  are  brought  together,  as  shown  by  Fig.  77,  the  lines  of 
force  are  supposed  to  flow  from  the  north  to  the  south  pole  and 
constitute  a  magnetic  field.  The  air  is  a  bad  conductor  of  these 
lines  of  force,  and  if  the  poles  are  wide  apart  a  weak  field  results. 
Soft  iron,  on  the  other  hand,  is  a  very  good  conductor,  and  if  a 
portion  of  the  space  between  the  poles  of  a  magnet  is  filled  with 
soft  wrought  iron,  it  assists  the  lines  of  force  to  flow,  and  the 
space  left  unbridged  has  a  stronger  field  than  would  be  the  case 
without  such  partial  bridging. 

A  horse-shoe  magnet  of  the  kind  described  is  called  a  per- 
manent magnet.  A  much  more  powerful  magnet  can  be  made 
by  passing  an  electric  current  round  and  round  a  bar  of  soft  iron 
or  high  permeability  steel,  and  such  electrically-excited  magnets 
are  used  in  practice,  excepting  for  the  very  smallest  class  of 
dynamos,  which  are  then  usually  known  as  magnetos. 

13 


194 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


If  a  wire  is  placed  between  the  legs  of  the  magnet,  as  shown 
by  Fig.  77  (the  black  dot  represents  the  wire  in  section),  and  is 
moved  sharply  downwards  so  that  it  cuts  the  lines  of  magnetic 
force,  an  electromotive  force  (E.M.F.)  is  induced  in  the  wire, 
and  if  the  ends  of  the  wire  are  joined  so  that  the  circuit  is  com- 
pleted, a  current  of  electricity  flows  round  it.  The  E.M.F., 
which  may  be  looked  upon  as  electrical  pressure,  is  spoken  of  as 
so  many  volts ;  the  amount  of  current  which  will  flow  through 
the  wire  (if  the  ends  are  joined)  depends  upon  the  E.M.F.  and 
the  resistance  of  the  wire. 

With  a  permanent  steel  magnet  and  single  wire  the  E.M.F. 
induced  would  be  too  small  to  produce  a  serviceable  current,  and 
obviously  it  would  be  very  inconvenient  to  move  the  wire 
rapidly  up  and  down  as  shown.  In  actual  practice  an  electrically- 
excited  magnet  or  number  of  magnets  are  used,  and  the  wire,  or 


Fig.  77  and  Ha. — Horse-shoe  magnets. 

wires,  which  have  to  cut  the  lines  of  force  are  placed  on  the 
outside  of  a  drum,  as  shown  by  Fig.  11  a.  The  drum  is  made  of 
soft  iron  plates  so  as  to  facilitate  the  passage  of  the  lines  of 
force ;  by  rotating  the  drum  rapidly  the  wires  on  its  outside  pass 
through  a  strong  magnetic  field,  or,  in  other  words,  cut  a  large 
number  of  lines  of  force  flowing  at  right  angles  to  the  wires. 
Such  a  drum  with  wires  is  called  an  armature,  although  an 
armature  need  not  necessarily  take  the  form  of  a  drum.  It 
should  be  mentioned  here  that  in  order  to  produce  E.M.F.  in  a 
wire  the  latter  must  pass  from  a  weak  field,  or  no  field,  into  a 
strong  field  or  vice  versd;  or,  if  moved  continuously  in  a  field 
of  the  same  strength,  the  speed  must  be  varied,  otherwise  no 
E.M.F.  will  be  obtained. 


ELECTRICAL    CHAPTER.  195 

The  amount  of  E.M.F.  produced  in  a  single  wire  on  a  drum, 
when  rotated  in  a  magnetic  field,  depends  upon  the  strength  of 
the  field  (or  number  of  lines  of  force  cut  by  the  wire),  and  the 
rapidity  with  which  the  wire  is  moved.  Assuming  that  one  wire 
cuts  sufficient  lines  of  force  at  a  speed  sufficiently  great  to 
produce  an  E.M.F.  of  1  volt,  then,  if  the  drum  is  wound  so 
that  there  are  100  turns  of  wire  round  it,  and  it  is  rotated  at  the 
same  speed  as  before,  an  E.M.F.  of  100  volts  will  be  produced, 
as  the  voltage  in  each  turn  of  the  wire  adds  itself  to  the  voltage 
produced  by  the  other  turns.  The  wire  must,  of  course,  be 
insulated  and  wound  in  the  right  way. 

We  have  said  that  the  amount  of  current  which  will  flow  in  a 
wire  is  dependent  upon  the  E.M.F.  and  the  resistance  of  the  wire, 

rlj 

or,  expressed  as  a  formula,  C  =  —      The  amount  of  current  is 

expressed  in  amperes,  and  the  resistance  of  a  wire  in  ohms,*  or 
parts  of  an  ohm,  so  that  the  formula,  translated  into  actual 

working  terms,  is  amperes  =  — r- Thus,  assuming  the  E.M.F. 

in  a  wire  is  100  volts,  and  the  resistance  when  the  circuit  is 
completed  is  2  ohms,  we  shall  have  a  current  of  50  amperes 
flowing.  If,  on  the  other  hand,  we  have  an  E.M.F.  of  1  volt  and 
a  resistance  of  2  ohms,  we  shall  only  have  a  current  of  half  an 
ampere  flowing. 

To  return  to  the  dynamo.  If  a  single  wire  is  used,  as  shown 
in  black  by  Fig.  77a,  when  the  drum  is  rotated  and  the  wire  cuts 
the  lines  of  force  by  descending  through  them  on  the  left-hand 
side,  the  current  will  flow  towards  the  spectator ;  when  the  wire 
cuts  the  lines  of  force  on  the  right-hand  side  by  rising  through 
them,  the  current  will  flow  away  from  the  spectator.  Thus  the 
current  in  the  wire  is  reversed  during  every  revolution  of  the 
drum,  and  is  called  an  alternating  current.  A  dynamo  arranged 
with  the  wires  or  conductors  arranged  to  give  this  effect  is  called 
an  alternating  current  generator,  or  alternator. 

If,  instead  of  a  single  wire  on  one  side  of  the  drum  only, 
the  wire  is  prolonged  and  wound  round  the  drum,  as  shown  by 
dotted  lines  in  Fig.  7  7  a,  it  will  at  once  be  seen  that  the  current 
flowing  towards  the  spectator  on  the  left-hand  side  and  away 
from  him  on  the  right-hand  side  will  produce  a  continuous 
current  in  the  encircling  wire,  but  the  direction  of  the  current 
will  be  reversed  once  during  every  revolution.  If,  however,  the 
current  flowing  round  the  armature  can  always  be  tapped  or 

*  Table  xvin.,  giving  the  resistance  of  wires  of  various  sections  in  ohms, 
will  be  found  at  the  end  of  this  chapter. 


196  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

drawn  off  at  the  point  where  its  E.M.F.  is  highest,  and  where  the 
current  is  flowing,  say  towards  the  spectator,  and  returned  to  the 
armature  at  a  point  where  the  current  is  flowing  away  from  the 
spectator,  a  continuous  or  direct  current  will  be  obtained,  and 
will  be  available  for  use  outside  the  dynamo.  This  tapping  of 
the  current  at  the  right  place  is  effected  by  means  of  a  com- 
mutator. The  commutator  consists  of  a  number  of  segments  of 
copper  insulated  from  each  other,  and  from  the  shaft ;  to  each  one 
of  these  segments  one  end  of  a  wire,  wound  round  the  armature, 
is  connected,  the  other  end  being  connected  to  the  next  segment. 
In  actual  practice  the  wire  is  taken  a  good  many  times  round 
the  drum  before  its  ends  are  connected  to  the  commutator 
segment.  Brushes  consisting  of  copper  gauze  pressed  tightly 
together,  or  of  carbon,  are  used  for  collecting  the  current.  One 
brush  or  set  of  brushes  is  placed  over,  and  pressed  by  a  spring 
upon,  the  segment  of  the  commutator  where  the  current  is  at  the 
greatest  E.M.F.,  and  where  it  is  flowing  in  the  right  direction ; 
another  set  of  brushes  returns  the  current  to  a  segment  at 
the  opposite  of  the  commutator,  thus  completing  the  circuit.  A 
dynamo  provided  with  such  a  commutator  is  called  a  continuous- 
current  or  direct-current  generator.  If  a  large  number  of  poles 
are  used,  then  a  set  of  brushes  is  provided  to  collect  the  current 
from  the  commutator  at  each  pole. 

We  have  seen  that  the  volt  is  the  unit  of  E.M.F.,  the  ampere 
the  unit  of  current,  while  the  ohm  is  the  unit  of  resistance  to 
the  passing  of  the  current.  One  ampere  of  current  at  a  pressure 
of  1  volt  is  called  1  watt.  Thus  5  amperes  at  a  pressure  of  2 
volts  =  10  watts,  or  2  amperes  at  a  pressure  of  5  volts  are  also 
10  watts.  A  thousand  watts  are  called  1  kilowatt,  and  dynamos 
are  usually  spoken  of  as  giving  so  many  kilowatts.  Thus  a 
dynamo  which  gives  an  output  of  100  amperes  at  100  volts  is  a 
10-kilowatt  dynamo.  A  dynamo  which  gives  1,200  amperes  at 
100  volts  is  a  120-kilowatt  dynamo  ;  while  a  dynamo  which  gives 
3,000  amperes  at  500  volts  is  a  1,500-kilowatt  machine. 

Electrical  current  cannot  be  produced  without  the  expenditure 
of  energy,  and  it  may  be  useful  at  this  stage  to  see  what  relation 
the  electrical  units  bear  to  a  mechanical  horse-power.  The  rela- 
tion is  this — 746  watts  =  1  electrical  horse-power.  An  electrical 
horse-power  is,  theoretically,  the  equivalent  of  1  mechanical  horse- 
power, and  if  there  were  no  losses  in  a  dynamo,  1  brake  horse- 
power transmitted  to  it  would  produce  746  watts.  But,  as 
a  fact,  there  is,  and  must  be,  a  certain  loss  of  energy  in  every 
dynamo.  In  the  first  place,  there  is  the  resistance  of  the  wires 
or  coils  carrying  the  current  round  the  armature,  and,  in  the 
second  place,  there  is  the  loss  of  that  portion  of  the  current. 


ELECTRICAL    CHAPTER.  197 

which  is  used  for  exciting  the  magnets.  There  are  also  the 
losses  due  to  mechanical  friction  and  windage. 

In  a  well-designed  dynamo  of,  say,  200  kilowatts  and  upwards, 
these  losses  may  not  exceed  6  per  cent.,  so  that  for  every  100 
brake  horse-power  transmitted  to  the  dynamo,  the  latter  will 
give  an  output  of  94  E.H.P. ;  in  this  case  the  efficiency  of  the 
dynamo  is  said  to  be  94  per  cent.  In  small  dynamos  such  a 
high  efficiency  is  not  obtained,  while  in  very  large  dynamos  the 
efficiency  may  be  greater. 

A  continuous-current  dynamo  can  be  used  either  for  generating 
electricity  or  it  can  be  used  for  converting  electrical  energy  back 
into  mechanical  work ;  when  a  dynamo  is  used  for  the  latter 
purpose,  it  is  called  a  motor.  The  electrical  losses  in  a  motor  are 
the  same  as  in  a  dynamo,  but  in  a  small  motor  of,  say,  10  H.P., 
one  would  not  expect  an  efficiency  higher  than  88  or  90  per  cent. 

It  may  be  of  interest  to  see  how  far  an  electrical  horse-power 
will  go  in  the  way  of  current  for  lighting  purposes.  A  16-candle- 
power  200  to  250  volt  incandescent  (glow  lamp)  requires,  when 
new,  about  60  watts  per  hour,  so  that  12J  lamps  will  absorb 
about  1  E.H.P  (746  watts);  124  lamps  will  absorb  10  E.H.P.; 
and  1,244  lamps  100  E.H.P.  The  student  must  not,  however, 
fall  into  the  mistake  of  thinking  that  a  steam  engine  indicat- 
ing 100  H.P.  is  capable  of  driving  a  dynamo  when  supplying 
current  for  1,244  lamps,  as  the  losses  already  referred  to  must 
be  taken  into  account.  In  the  first  place,  the  loss  due  to 
friction  in  the  engine  itself  will  probably  reduce  the  available 
power  to  90  B.H.P.  Then,  assuming  the  efficiency  of  the  dynamo 
is  92  per  cent.,  we  shall  have  only  92  per  cent,  of  90  B.H.P. ,  or 
82-8  E.H.P.,  to  dispose  of.  If  another  5  per  cent,  (say  4-2  H.P.) 
is  lost  through  the  resistance  of  the  wires  or  leads  conveying  the 
current  to  the  lamps,  we  have  only  78-6  E.H.P.,  or  58,635  watts, 
to  dispose  of.  Assuming  each  lamp  requires  60  watts,  then  a 
100  I. H.P.  engine  coupled  to  a  dynamo  will  supply  current  for 
977  lamps.  In  practice  it  is  usually  reckoned  that  1  I. H.P.  is 
required  for  8  to  10  16-candle-power  incandescent  lamps. 

Arc  lamps,  or  those  in  which  the  light  is  caused  by  the 
current  tearing  away  and  making  incandescent  small  particles 
of  carbon  in  jumping  from  one  carbon  to  another,  use  less 
current  per  candle-power  than  those  of  the  incandescent  form, 
but  the  candle-power  of  an  arc  lamp  is,  of  course,  immensely 
greater  than  that  of  a  glow  lamp.  The  E.M.F.  required  for 
each  arc  lamp,  if  used  singly,  is  about  50  volts,  but  five  lamps 
may  be  placed  in  series  on  a  240-volt  circuit.  The  current  used 
is  from  5  to  20  amperes,  depending  upon  the  size  of  the  carbons 
and  candle-power  of  the  lamps. 


198  MECHANICAL    ENGINEERING   FOR    BEGINNERS. 

A  2,000  candle-power  arc  lamp  of  ordinary  or  open  type 
absorbs  about  500  watts,  so  that  in  actual  practice  the  engine 
driving  the  generator  must  indicate  nearly  1  horse-power  for 
every  2,000  candle-power  arc  lamp. 

The  carbons  in  an  ordinary  open  type  of  arc  lamp  require  to  be 
replaced  after  burning  for  12  to  18  hours,  but  in  the  closed  form 
of  arc  lamp,  such  as  the  Jandus,  where  the  arc  is  enclosed  and 
is  maintained  in  an  atmosphere  of  carbon  monoxide  gas  and 
nitrogen,  the  carbons  will  last  for  120  to  200  hours,  depending 
upon  the  size  of  the  carbons  and  the  amount  of  current  sent 
through  them.  From  5-5  to  7  amperes  at  100  to  120  volts  are 
required  with  this  form  of  lamp. 

To  return  once  more  to  the  generator.  In  the  earlier  stages 
of  electric  development,  the  two-pole  horse-shoe  form  of  dynamo 
was  considered  the  best  for  machines  giving  an  output  up  to 
200  kilowatts,  but  when  larger  machines  than  this  were  called 
for,  it  was  found  that  less  material  was  required  in  dynamos 
having  a  considerable  number  of  poles,  and  multipolar  machines 
have  now  generally  superseded  bipolar  machines  even  in  the 
smaller  sizes. 

Fig.  78  gives  the  end  view  of  a  750-kilowatt  multipolar 
dynamo  without  its  end  bearing.  In  a  multipolar  dynamo,  as 
shown,  the  yoke  and  magnets  are  made  of  high  permeability 
cast  steel — i.e.,  steel  which  presents  very  small  resistance  to  the 
lines  of  magnetic  force.  The  poles  being  alternately  north  and 
south,  the  lines  of  magnetic  force  do  not  require  to  travel  right 
across  the  armature  as  in  a  two-pole  dynamo,  but  merely  from 
one  north  pole  through  a  portion  of  the  soft  iron  core  of  the 
armature,  and  into  the  adjoining  south  pole. 

In  a  direct-current  generator  giving  current  at  a  high  voltage, 
it  is  necessary  to  have  a  large  number  of  segments  in  the  com- 
mutator in  order  to  keep  the  difference  of  potential  between 
each  segment  as  small  as  possible,  and  thus  avoid  any  danger 
of  the  current  jumping  across  the  insulation  between  the  seg- 
ments. If  the  commutator  is  of  large  diameter  and  the  speed  is 
high,  the  centrifugal  force  of  the  segments  is  considerable.  If, 
on  the  other  hand,  the  diameter  of  the  commutator  is  small, 
there  is  a  risk  of  the  current  jumping  from  one  set  of  brushes 
to  the  next.  Difficulties  in  connection  with  the  commutators 
of  direct-current,  high-voltage  generators  are  not  uncommon, 
especially  when  such  generators  are  coupled  to  steam  turbines. 

Direct  current  at  a  high  E.M.F.  can  be  obtained  by  working 
two  or  more  dynamos  in  series — i.e.,  the  first  dynamo  supplies 
current  to  the  second,  the  second  to  the  third,  and  so  on.  Thus 
two  dynamos,  each  giving  50  amperes  at  3,000  volts,  will,  if 


ELECTRICAL    CHAPTER. 


199 


coupled  up  in  series,  give  50  amperes  at  6,000  volts,  and  three 
such  machines  50  amperes  at  9,000  volts.  This  system  was  first 
carried  out  on  a  practical  scale  by  M.  Thury,  and  is  known  as 
the  Thury  system. 

The  reason  why  high  voltages  are  required  is  this — a  current 
of  1,000  amperes  at  a  pressure  of  10  volts  has  the  same  energy 
as  a  current  of  10  amperes  at  a  pressure  of  1,000  volts,  but  the 
former  current  requires  a  conductor  having  about  J  of  a  square 
inch  sectional  area  to  carry  it,  while  a  current  of  10  amperes 


Y,  Yoke  or  magnet  ring.  C,  Commutator. 

B,  Part   of  brush  ring  carrying  S,  Shaft, 

three  brushes. 

Fig.  78. — Twelve-pole  direct-current  dynamo. 

requires  a  conductor  having  only  TJ^  part  of  a  square  inch  to 
carry  it,  and  as  copper  is  very  expensive,  it  is  necessary,  when 
current  has  to  be  transmitted  to  a  great  distance,  to  transmit  it 
at  as  high  voltage  and  small  amperage  as  possible. 

An  alternator  is  much  more  suitable  for  producing  current  at 
a  high  E.M.F.  than  a  direct-current  generator,  as  it  requires  no 
commutator.  An  alternating  current  can  be  used  for  lighting 


200  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

purposes,  provided  the  alternations  are  sufficiently  rapid — viz., 
from  40  to  50  per  second ;  an  alternating  current  also  has  the 
following  valuable  property : — If  two  conductors  are  placed 
side  by  side,  and  an  alternating  current  is  sent  through  one 
of  them,  it  will  induce  a  current  (having  alternations  in  the 
opposite  direction)  in  the  other  conductor,  and  the  E.M.R  of 
the  secondary  or  induced  current  can  be  made  either  higher 
or  lower  than  that  of  the  primary  current.  Thus,  if  a  wire 
of  large  diameter,  carrying  a  current  of  a  good  many  amperes, 
is  placed  alongside  a  smaller  wire,  and  having  a  greater 
resistance,  the  current  induced  in  the  latter  will  be  of  higher 
voltage,  but  of  smaller  amperage,  than  in  the  primary  wire, 
and  vice  versa. 

By  the  aid  of  a  suitable  transformer,  an  alternating  current 
can  therefore  be  transformed  either  to  a  higher  or  lower  E.M.F. 
A  continuous  or  direct  current  will  only  induce  a  current  in  a 
second  wire  at  starting  and  stopping.  In  lighting  a  town  having 
a  number  of  outlying  districts,  it  is  customary  to  generate  an 
alternating  current  at  a  high  E.M.F.,  and  to  transmit  at  this 
high  tension  to  a  number  of  transformer  stations,  where  the 
current  is  transformed  down  to  the  E.M.F.  at  which  it  will  be 
used  on  the  consumer's  premises.  The  Board  of  Trade  will  not 
allow  the  E.M.F.  of  any  current  entering  a  private  house  or  shop 
to  exceed  500  volts.  As  a  rule,  the  E.M.F.  of  the  current  used 
in  a  private  house  or  shop  does  not  exceed  220  or  240  volts. 

If  a  current  has  to  be  transmitted  to  a  distance  of  many  miles, 
and  the  voltage  decided  upon  is  too  high  for  the  insulation  of 
the  alternator,  the  current  can  be  generated  at  a  moderate 
E.M.F.  and  then  transformed  up  (by  a  step-up  transformer), 
transmitted  to  its  destination,  and  then  transformed  down 
again. 

The  insulation  of  conductors  carrying  current  at  a  very  high 
E.M.F.  can  be  more  easily  arranged  in  a  stationary  transformer 
than  in  the  generator  itself,  but  there  is  a  loss  every  time  the 
current  is  transformed  up  or  down. 

Alternators,  Single  and  Multi- phase. — In  a  large  alter- 
nator, where,  as  we  have  seen,  it  is  not  necessary  to  have  a 
commutator,  it  is  customary  to  rotate  the  field  magnets,  and 
to  have  a  fixed  armature.  A  large  number  of  poles,  alternately 
north  and  south,  are  arranged  on  the  rotating  magnet,  while  the 
armature  is  outside  it.  Fig.  79  shows  the  arrangement  diagram- 
matically  with  a  uni-coil  winding — i.e.,  one  slot  or  coil  per  pole 
piece.  Plan  1  shows  the  direction  of  the  current  in  one  of  the 
armature  coils  while  a  north  pole  is  passing  under  the  right- 
hand  portion  of  the  coil,  while  Plan  2  shows  the  reversal  of  the 


ELECTRICAL    CHAPTER. 


201 


current   while   a   south    pole    is    passing   under   the   left-hand 
portion  of  the  coil. 


Fig.  79.— Alternator. 

Two-  and  Three-phase  Alternators. — An  ordinary  single- 
phase  alternating  current  has  one  great  disadvantage — viz.,  it 
will  not  start  an  electric  motor.  This  difficulty  in  connection 


202 


MECHANICAL    ENGINEERING   FOR    BEGINNERS. 


with  the  alternating  current  has,  however,  been  overcome  by 
employing  three-phase  alternators  and  motors.     The  meaning  of 


Fig.  81. 


Fig.  82. 


Fig.  83. 
Alternating  current  diagrams. 

a  three-phase  alternating  current  can  best  be  explained  by  the 
help  of  diagrams. 


ELECTRICAL    CHAPTER. 


203 


Fig.  80  represents  diagrammaticallya  simple  form  of  single-phase 
alternator  having  a  north  and  south  pole,  and  a  single  conductor 
on  the  drum,  and  Fig.  81  represents  graphically  the  alternating 
current  produced  by  one  revolution  of  the  drum  carrying  the 


Fig.  84. — Alternate  current  diagram. 

conductor;  the  highest  point  of  the  curve  represents  the  greatest 
E.M.F.  of  the  current,  and  the  lowest  point  the  zero.  One 
revolution  of  the  drum  produces  one  complete  period,  so  that,  if 
the  drum  revolves  50  times  in  a  second,  the  periodicity  of  the 
alternator  (having  one  pair  of  poles)  will  be  50  complete  periods 


204 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


per  second.  If  a  second  conductor  is  placed  on  the  drum  the 
current  produced  will  be  as  shown  by  Fig.  82,  and  will  be  a 
two-phase  current.  If  the  number  of  revolutions  of  the  drum 
remains  the  same  as  before,  the  periodicity  of  the  two-phase 
current  will  also  remain  unchanged — viz.,  50  per  second.  If  a 
third  conductor  is  placed  on  the  drum  the  resulting  current  will 
be  a  three-phase  current,  and  will  be  as  shown  by  Fig.  83. 

Instead  of  two  poles,  as  shown,  there  are,  as  previously 
mentioned,  usually  a  large  number  of  poles;  these  poles  are 
rotated  while  the  armature  is  stationary.  The  arrangement  of  a 
uni-coil  three-phase  alternator  is  shown  by  Fig.  84.  In  actual 


Fig.  85. — Alternating  current  generator. 

practice,  however,  the  two  legs  of  the  U-coil  are  seldom  separated 
so  widely  as  to  come  one  over  a  North  pole  and  one  over  a 
South  pole.  The  winding  of  a  three-phase  alternator  varies 
greatly,  but  Fig.  85  is  a  good  example  of  such  winding. 

Periodicity  of  Alternating  Currents. — The  periodicity  of 
an  alternating  current,  if  the  number  of  poles  and  number  of 
revolutions  of  the  alternator  are  known,  may  be  found  thus  : — 

P  =  N  x  R; 

where  P  =  periodicity  per  second. 

N  =  number  of  pairs  of  poles. 
B  =  revolutions  per  second. 


ELECTRICAL    CHAPTER.  205 

If  it  is  desired  to  find  the  number  of  revolutions  per  minute 
an  engine  or  turbine  must  make  to  give  a  certain  periodicity  per 
second,  the  formula  is — 

Fx60. 

T^ 

where  R  =  revolutions  per  minute. 
P  =  periodicity  per  second. 
1ST  =  number  of  pairs  of  poles. 

Example. — Suppose  an  alternator,  which  has  to  be  driven  by  a  turbine, 
has  4  poles  (2  pairs),  and  the  periodicity  required  is  50  per  second,  at  what 
speed  must  the  turbine  run  ?  The  calculation  is — 

50  x  60 
— - —  —  1,500  revs,  per  minute. 

Or,  suppose  an  alternator,  requiring  to  be  driven  by  a  high-speed 
engine,  has  20  poles  (10  pairs),  and  the  periodicity  required  is  50  per 
second,  at  what  speed  must  the  engine  run  ? 

30  *Q60  =  300  revs,  per  minute. 

Before  leaving  the  subject  of  generators,  it  may  be  well  to 
explain  the  difference  between  series-wound,  shunt-wound,  and 
compound-wound  dynamos.  A  series- wound  dynamo  is  one  in 
which  the  whole  of  the  current  generated  in  the  armature  is 
passed  round  the  magnets  to  excite  them.  Such  a  dynamo  is 
seldom  met  with,  as,  if  extra  resistance  is  put  into  the  main 
circuit  through  which  the  current  is  flowing,  the  current  will  be 
reduced,  and  the  exciting  current  will  also  be  reduced  ;  the  same 
thing  happens  if  the  speed  of  the  dynamo  falls  off.  In  both 
these  cases  the  exciting  current  is  weakened  just  when  it  should 
really  be  strengthened.  Series-wound  motors  are,  however,  used, 
as  a  strong  field  is  required  at  starting. 

A  shunt-wound  dynamo  is  one  in  which  the  current  is  split  up 
when  it  leaves  the  collecting  brushes ;  a  portion  of  the  current 
is  sent  round  the  magnets,  and  the  remainder  flows  into  the  main 
circuit.  If  extra  resistance  is  put  into  the  main  circuit,  the 
current  finds  an  easier  path  round  by  the  field  magnets,  and  so 
strengthens  the  field.  The  amount  of  current  flowing  round  the 
field  magnets  is  usually  regulated  by  means  of  a  resistance  and 
switch ;  thus  the  strength  of  the  field  can  be  increased  or 
decreased  at  will. 

A  compound-wound  dynamo  is  one  in  which  a  few  turns  of 
series  winding  are  placed  over  the  shunt  winding,  and  arranged 
to  act  in  opposition  to  it.  Thus,  if  the  speed  of  the  generator 
should  fall  and  the  main  current  be  reduced,  the  opposing  power 


206  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

of  the  series  winding  will  also  be  reduced,  and  the  shunt  wind- 
ing, freed  from  this  opposition,  will  more  strongly  excite  the 
magnets. 

A  separately-excited  dynamo  is  one  in  which  the  exciting 
current  is  supplied  from  another  dynamo,  or  from  some  inde- 
pendent source.  All  alternators  require  to  be  separately 
excited. 

Hotary  Converter  and  Motor  Generator. — If  alternating 
current  is  employed  for  transmitting  energy  to  a  distance,  and 
it  is  required  to  convert  the  alternating  current  into  continuous 
or  direct  current,  this  can  be  done  in  two  ways.  The  first  is 
by  means  of  a  rotary  converter.  This  consists  of  a  motor,  the 
armature  of  which  is  driven  by  the  alternating  current ;  over  the 
winding  required*  for  this  alternating  current  is  placed  winding 
suitable  for  producing  continuous  current.  When  the  armature 
is  rotated,  the  latter  winding  produces  direct  current  in  the  same 
way  as  in  a  direct-current  generator. 

A  motor  generator  consists  of  an  alternating-current  motor 
coupled  to  and  driving  a  continuous-current  generator.  A  motor 
generator  is  practically  the  same  thing  as  a  rotary  transformer, 
only  the  alternating-current  motor  and  the  direct-current  gen- 
erator are  two  distinct  machines  coupled  together,  instead  of 
being  combined,  as  in  the  case  of  the  rotary  transformer.  It  is 
found  that  better  regulation  of  the  E.M.R  can  be  obtained  by 
the  use  of  two  separate  machines. 

Primary  Batteries  and  Accumulators. — A  small  current 
at  a  low  E.M.F.  can  be  obtained  from  primary  batteries  or  cells, 
but  these,  although  useful  for  electric  bells  and  telephone  work, 
hardly  concern  the  mechanical  engineer,  as  the  production  of 
current  on  a  large  scale  by  batteries  is  out  of  the  question  on 
account  of  the  cost. 

The  simplest  form  of  cell,  known  as  a  Daniell  cell,  consists  of  a 
plate  of  zinc  and  one  of  copper  immersed  in  dilute  sulphuric  acid; 
the  acid  eats  away  the  zinc,  and,  if  the  zinc  and  copper  plates  are 
connected  by  a  wire,  a  current  flows  from  one  plate  to  the  other 
through  the  acid  and  through  the  wire.  The  E.M.F.  of  such  a 
cell  is  1  *05  volts.  In  the  Leclanche  cell,  largely  used  for  electric 
bells,  there  is  a  rod  of  zinc,  also  a  porous  pot  containing  a  block 
of  carbon,  small  pieces  of  crushed  carbon,  and  black  oxide  of 
manganese ;  these  replace  the  copper  plate  of  the  Daniell  cell. 
The  zinc  and  porous  pot  are  immersed  in  a  solution  of  sal- 
ammoniac.  The  E.M.F.  of  this  cell  is  14  volts. 

Accumulators. — Accumulators  are  used  for  storing  electricity, 
if  such  an  expression  may  be  used,  for  the  action  is  really  a 
chemical  one.  An  accumulator  consists  of  two  or  more  plates, 


ELECTRICAL    CHAPTER.  207 

positive  and  negative,  pierced  by  a  large  number  of  holes.  The 
holes  in  the  negative  plate  are  usually  filled  with  pellets  of  lead 
oxide,  while  the  holes  in  the  positive  plates  are  filled  with  pellets 
of  peroxide  of  lead ;  these  plates  are  immersed  in  dilute  sulphuric 
acid ;  a  current  is  passed  from  one  plate  to  the  other  through  the 
acid,  when  a  certain  chemical  change  takes  place.  After  the 
current  has  passed  for  a  sufficient  length  of  time,  the  accumulator 
is  said  to  be  charged.  That  is  to  say,  if  the  plates  are  connected 
together  by  a  wire,  a  current  will  flow  in  the  direction  opposite 
to  that  in  which  the  current  entered  the  accumulator.  Each  cell 
is  charged  until  the  E.M.F.  reaches  about  2-5  or  2-6  volts ;  when 
the  charging  wires  are  disconnected,  the  E.M.F.  falls  to  about 
2-1  volts,  and  as  the  accumulator  discharges  its  current  the 
E.M.F.  gradually  falls  to  1*85  volts,  when  the  accumulator  is 
practically  discharged.  Any  further  discharge  is  injurious  to 
the  accumulator. 

The  capacity  of  an  accumulator  in  amperes  depends  upon  the 
surface  of  the  plate  or  plates.  In  practice  there  are  several 
plates  in  each  cell ;  all  the  positive  plates  are  connected  to  one 
bar,  and  all  the  negative  plates  to  another  bar.  To  obtain  a 
higher  E.M.F.  than  2*1  volts,  a  number  of  accumulators  may 
be  placed  in  series ;  in  this  case  the  positive  terminal  of  one 
accumulator  is  connected  to  the  negative  terminal  of  the  next, 
and  so  on.  Thus,  if  two  accumulators  are  placed  in  series, 
the  E.M.F.,  when  charged,  is  4'2  volts* ;  if  10  accumulators  are 
placed  in  series,  the  E.M.F.  is  21  volts;  and  if  50  are  in  series, 
the  E.M.F.  is  105  volts.  If  it  is  essential  to  maintain  the  total 
E.M.F.  constant  while  the  accumulators  are  being  discharged,  a 
few  additional  cells  must  be  added  to  the  series  to  compensate 
for  the  falling  off  of  E.M.F. 

In  order  to  charge  accumulators,  direct  current  must  of  course 
be  used,  and  the  E.M.F.  of  the  current  must  be  a  little  higher 
than  that  of  the  whole  of  the  cells  being  charged  in  series. 
Accumulators  may  be  coupled  up  in  parallel ;  this  means  that 
the  whole  of  the  positive  plates  are  connected  together,  and  the 
whole  of  the  negative  plates  together.  If  coupled  up  in  this 
way,  the  total  E.M.F.  is  only  that  of  a  single  cell,  but  the  total 
current  in  amperes  is  increased  in  proportion  to  the  number  of 
cells  employed. 

The  capacity  of  an  accumulator  is  known  by  ampere-hours ; 
thus  a  24  ampere-hour  accumulator  will  give  1  ampere  for 
twenty-four  hours,  or  2  amperes  for  twelve  hours,  or  4 
amperes  for  six  hours.  The  rate  of  charging  and  discharging 

*  The  4-volt  accumulators  used  for  motor-car  ignition  are  really  two 
2'1  cells  coupled  in  series  in  one  case. 


208  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

is  stated  by  the  makers,  and  should  not  be  greatly  exceeded ; 
if  it  is,  gas  is  formed  too  quickly,  and  tends  to  dislodge  the 
pellets. 

The  acid  used  consists  of  8  parts  of  sulphuric  acid  to  5 
parts  of  water,  and  has  a  density  of  1*2.  As  the  accumulator 
discharges  the  acid  becomes  weaker,  so  that  by  placing  a  hydro- 
meter in  the  acid  and  noting  how  far  it  sinks,  the  amount  of 
charge  in  the  accumulator  may  be  known  approximately. 

Accumulators  are  very  useful  for  country  house  lighting,  as 
they  can  be  charged  in  the  day  time,  and  the  current  used 
during  the  evening  and  night  time.  They  are  also  useful  in 
small  central  stations  as  a  reserve,  and  to  maintain  a  supply  of 
current  during  the  night,  should  the  demand  not  be  sufficient  to 
warrant  the  running  of  an  engine  and  dynamo.  Accumulators 
are  also  used  sometimes  as  a  steadier  for  the  current ;  in  this 
way, — the  engine  and  dynamo  supply  the  main  demand  for  cur- 
rent, any  excess  current  going  through  the  accumulators ; 
should  the  demand  be  suddenly  greater  than  the  engine  and 
dynamo  are  capable  of  meeting  and  the  E.M.F.  falls,  then  the 
accumulators  automatically  discharge  their  current,  and  so 
assist  the  engine  and  dynamo.  When  the  demand  falls  off 
and  the  E.M.F.  rises,  some  of  the  current  from  the  dynamo 
flows  round  by  the  accumulators  and  recharges  them. 

The  efficiency  of  an  accumulator  is  from  70  to  80  per  cent. — • 
that  is  to  say,  it  will  return  70  or  80  per  cent,  of  the  current  put 
into  it. 

The  disadvantages  of  accumulators  are — (1)  Their  first  cost. 
This  is  somewhat  high,  as  for  an  E.M.F.  of  120  volts  about  70 
accumulators  are  required,  as  one  can  only  reckon  upon  the 
E.M.F.  of  the  cells  when  nearly  discharged — viz.,  1-85  volts; 
(2)  the  cost  of  upkeep,  which  is  rather  high ;  and  (3)  their  great 
weight.  Objections  2  and  3  prevent  accumulators  coming  into 
general  use  for  traction  purposes. 

The  instruments  used  for  measuring  the  electric  current  are 
the  voltmeter,  for  measuring  the  E.M.F. ;  the  ammeter  (or 
ampere  meter),  for  measuring  the  amount  of  current  flowing  ; 
and  the  wattmeter ;  the  latter  measures  the  volts  multiplied  by 
the  amperes.  An  automatic  cut-out  is  an  instrument  for 
automatically  breaking  the  circuit  should  an  excessive  current 
pass. 

A  summary  of  the  electrical  units,  with  which  it  is  desir- 
able that  the  mechanical  engineer  should  be  familiar,  is  as 
follows : — 


ELECTRICAL    CHAPTER. 


209 


Ampere 
Volt 

Ohm 

1  ampere  x  1  volt 
746  watts 
1,000  watts 
1  megohm 

Volts 

^ =  amperes. 

Ohms 

Amperes 


unit  of  current. 

unit  of    pressure   or  electromotive 

force  (E.M.F.). 
unit  of  resistance. 
1  watt. 

1  electrical  horse-power. 
1  kilowatt, 
one  million  ohms. 


Volts 
Amperes 
ohms  =  volts. 


ohms. 


The  following  table  giving  the  resistances  in  ohms  per  1,000 
yards  of  copper  wires  of  different  diameters,  and  the  amount  of 
current  in  amperes  carried,  may  possibly  be  useful.  The  current 
given  is  based  upon  4,000  amperes  per  square  inch  of  sectional 
area.  The  current  carried  by  a  cable  having  several  strands 
can  also  be  ascertained  by  the  table — thus,  a  -£$  cable  will  carry 
A  current  three  times  greater  than  a  single  No.  20  wire,  or 
12  amperes.  A  -^  cable  will  carry  28  amperes,  &c.  : — 


TABLE  XVIII. 


Working 

Resistance 

Current 

S.W.G. 

Diameter. 

Sectional  Area. 

in  1,000 

at  4,000 

Yards. 

Amperes 

• 

per  Sq.  In. 

Inch. 

Mm. 

Inch. 

Mm. 

Ohms 

Amperes. 

22 

•028 

•711 

•0006 

•397 

38-46 

2-4 

20 

•036 

•914 

•0010 

•657 

23-26 

4-0 

18 

•048 

1-22 

•0018 

1-167 

13-10 

7'2 

16 

•064 

1-62 

•0032 

2-075 

7-36 

12-8 

15 

•072 

1-83 

•0040 

2-627 

5-81 

16-0 

14 

•080 

2-03 

•0050 

3-243 

4-71 

20-0 

13 

•092 

2-34 

•0066 

4-287 

3-57 

26-6 

12 

•104 

2-64 

•0085 

5-48 

2-78 

34-0 

11 

•116 

2-95 

•0106 

6-818 

2-24 

42-4 

10 

•128 

3-25 

•0129 

8-302 

1-84 

51-5 

9 

•144 

3-66 

•0163 

10-507 

1-45 

65'2 

8 

•160 

4-06 

•0201 

12-972 

1-18 

80-0 

7 

•176 

4-47 

•0243 

15-695 

•99 

97-2 

6 

•192 

4-88 

•0289 

18-679 

•83 

115-6 

14 


Ill 


CHAPTER  XII. 
HYDRAULIC  MACHINERY. 

THE  words  "  hydraulic  machinery  "  cover  a  somewhat  wide  field : 
we  will  first  consider  hydraulic  machines,  such  as  presses, 
riveters,  and  lifts,  and  then  pass  on  to  water-wheels,  turbines, 
and  pumps. 

Power  can  be  transmitted  to  considerable  distances  by  means 
of  water  under  pressure  in  a  safe,  clean,  and  fairly  economical 
manner,  and  such  transmission  is  usually  employed  in  cases 
where  large  powers  are  required  to  be  exerted  during  a  short 
space  of  time  and  intermittently,  as  in  hydraulic  riveters, 
hydraulic  presses  for  flanging  boiler  plates,  &c.  Hydraulic 
power  is  also  largely  used  for  cranes  and  lifts  where  the  power, 
although  not  necessarily  great,  is  required  intermittently. 

The  principles  governing  the  construction  of  hydraulic 
machinery  in  which  water  under  high  pressure  is  employed 
are  fairly  simple,  none  of  the  problems  arising  in  connection 
with  the  use  of  steam,  such  as  initial  condensation,  re-evapo- 
ration, &c.,  are  met  with,  and  if  the  amount  of  power  required 
is  known  it  is  not  a  difficult  matter  to  design  a  machine  to 
give  it. 

The  hydraulic  press  fitted  with  a  small  hand  pump,  illustrated 
by  Fig.  86,  shows  the  principle  upon  which  most  hydraulic 
presses,  riveters,  and  lifts  work.  The  action  is  as  follows  : — 
The  ram  of  the  press  is  of  large  diameter  while  the  pump 
plunger  is  of  very  small  diameter.  The  small  plunger  on  being 
raised  draws  in  water  through  the  valve  at  the  bottom  of  the 
pump  j  on  being  pressed  down  the  plunger  forces  water  through 
the  valve  at  the  side  of  the  pump  into  the .  main  cylinder  which 
contains  the  ram  of  the  press.  Now,  if  the  area  of  the  ram  is 
.fifty  times  greater  than  that  of  the  small  plunger,  every  1-lb. 
pressure  exerted  on  the  plunger  will  give  a  pressure  of  50 
Ibs.  on  the  ram  :  if  a  pressure  of  100  Ibs.  is  exerted  on  the 
small  plunger  a  pressure  of  5,000  Ibs.  will  be  given  to  the 
large  ram. 

Of  course  the  large  ram  moves  very  slowly  if  the  water  is 
forced  into  the  cylinder  by  a  very  small  plunger,  and  in  practice 
such  a  hand  pump  would  not  be  used,  but  even  with  pumps 


212 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


driven  by  steam  power  the  motion  of  the  ram  would  be  too 
slow  if  the  water  were  pumped  directly  to  the  cylinder.  This 
difficulty  is  overcome  by  pumping  the  water  into  an  accumulator, 
storing  it  there  under  pressure,  and  drawing  from  the  accumu- 
lator as  required. 


Fig.  86. — Hydraulic  press. 

A  well-designed  hydraulic  accumulator  having  a  ram  8  inches 
in  diameter  and  a  stroke  of  14  feet  is  shown  by  Fig.  89. 
Drawings  of  accumulators  are  not  frequently  given  in  text 


Fig.  87. — Portable  hydraulic  riveter. 

books  or  technical  journals,  and  the  reader  would  do  well  to 
examine  the  illustration  carefully,  comparing  it  with  the  small 
drawing  of  an  accumulator  which  is  taken  from  a  work  of  a 


HYDRAULIC    MACHINERY. 


213 


popular  character.       The  accumulator  illustrated  on  the  larger 
scale  shows  the  simplicity  and  directness  characteristic  of  all  the 


Fig.  89. — Hydraulic  accumulator. 

designs  of  the  late  Mr.  Tweddell  (and  of  Messrs.  Fielding  & 
Platt).  It  will  be  seen  how,  by  inverting  the  cylinder,  the 
gland  has  been  made  accessible  for  packing,  how  the  weight 


214  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

in  the  casing  is  carried  by  the  cylinder  without  the  necessity 
for  supporting  it  from  a  cross  beam  at  the  top,  and  how  the 
cylinder  walls  form  the  inside  of  the  casing,  enabling  a  single 
outer  casing  to  take  the  place  of  the  inner  and  outer  casings 
necessitated  by  a  fixed  cylinder  and  moving  ram.  In  very  large 
accumulators  a  supporting  top  cross  girder  is  sometimes  used, 
but  the  moving  cylinder  is,  of  course,  retained.  To  proceed 
with  the  description: — The  accumulator  consists  of  a  cylinder 
which  is  free  to  move  up  and  down  on  a  ram;  the  cylinder  carries 
a  large  wrought-iron  casing  which  is  filled  with  scrap  iron,  slag, 
or  any  material  sufficiently  heavy  to  give  the  required  weight 
and  pressure  per  square  inch  on  the  water.  When  water  under 
pressure  is  pumped  into  the  accumulator  the  cylinder  carrying 
the  casing  rises,  and  the  water  in  the  accumulator  remains  under 
pressure  until  it  is  required.  When  water  is  taken  from  the 
accumulator  the  cylinder  and  casing  descend  for  a  short  distance, 
and  a  further  supply  of  water  is  pumped  in,  and,  as  the  capacity 
of  the  accumulator  is  much  greater  than  that  of  the  cylinder  of 
any  one  hydraulic  machine,  there  is  always  available  a  supply  of 
water  under  pressure. 

In  practice  steam-driven  pumps  are  usually  employed  to  force 
the  water  into  the  accumulator,  and  there  is  a  simple  device  by 
means  of  which  the  pumps  are  stopped  when  the  accumulator 
lias  reached  the  top  of  its  stroke,  and  automatically  started  when 
the  accumulator  has  Tallen  a  certain  distance. 

The  pressures  used  in  hydraulic  engineering  vary  from  750 
Ibs.  per  square  inch  to  2,240  Ibs.  per  square  inch.  The  pressure 
used  by  Mr.  Tweddell,  and  still  employed  by  Messrs.  Fielding 
&  Platt,  in  connection  with  hydraulic  riveters,  is  1,500  Ibs. 
per  square  inch. 

The  credit  for  the  introduction  of  hydraulic  riveting  machinery 
is  due  to  the  late  Mr.  R.  H.  Tweddell,  and  to  him  alone.  This 
gentleman  had  considerable  difficulty  in  getting  any  firm  of 
engineers  to  take  up  and  manufacture  his  plant.  Shipbuilders, 
boilermakers,  and  others  considered  that  it  was  impossible 
to  make  a  riveter  sufficiently  portable  to  be  of  much 
use,  and  they  doubted  whether  the  cost  of  closing  rivets  by 
hydraulic  power  would  be  as  low  as  was  the  case  with  hind- 
riveting.  However,  after  Messrs.  Fielding  &  Platt  undertook 
the  manufacture  of  the  riveters,  and  their  advantages  became 
known,  hydraulic  riveting  soon  became  general,  and,  at  the 
present  time,  probably  no  boiler  works  or  shipbuilding  yard  is 
without  a  hydraulic  riveting  plant.  In  addition  to  doing  the 
work  more  quickly  and  cheaply  than  by  hand,  the  work  done  is 
sounder  and  better.  The  comparison  of  a  section  cut  through 


HYDRAULIC    MACHINERY. 


215 


ar  riveted  joint,   the   rivets  of  which   have  been   hydraulically 
closed,  with  a  section  cut  through  a  pair  of  plates  and  through 


Fig.  90. —Hydraulic  riveter. 

rivets   which  have  been  closed  by  hand,   shows  the  difference 
at  once. 


216  MECHANICAL    ENGINEERING   FOR    BEGINNERS. 

Fig.  90  shows  a  large  fixed  riveter  and  plate  closer.  Water 
is  first  admitted  to  the  cylinder  A,  and  moves  forward  the  plate 
closing  cup  C  (shown  to  a  larger  scale  at  the  bottom  of  the 
illustration);  this  cup  presses  the  plates  firmly  together,  and 
holds  them  so  while  pressure  is  admitted  to  cylinder  D;  the 
effect  is  to  move  forward  the  die  H,  and  to  close  the  rivet. 

In  the  earlier  days  of  hydraulic  riveters  water  under  full 
pressure  was  used  for  moving  the  rams  through  the  whole  of  the 
outward  stroke,  the  small  ram  E  being  used  for  the  return 
stroke  only.  This  was  a  somewhat  wasteful  way  of  using  the 
water,  and  modern  riveters  are  either  supplied  with  water  under 
two  pressures,  the  lower  pressure  being  used  to  bring  the  die  up 
to  its  work,  and  the  full  pressure  for  closing  the  rivet;  or  the 
small  ram  is  used  for  bringing  the  die  up  to  its  work,  water 
being  permitted  to  flow  into  the  cylinder  from  a  tank  20  to  30 
feet  overhead,  and  the  full  pressure  being  used  for  doing  the 
actual  work. 

In  cases  where  it  is  inconvenient  to  bring  the  work,  such 
as  a  long  lattice  girder,  to  the  riveter,  portable  riveters  are 
employed.  A  portable  riveter  is  shown  by  Fig.  87.  It 
will  be  noticed  that  the  cylinder  is  placed  at  the  end  of  the 
riveter  farthest  from  the  dies ;  this  arrangement  admits  of 
the  dies  being  used  in  confined  spaces.  The  riveter  can  swivel 
at  A  and  B,  also  at  the  hook  C  by  which  it  is  hung  from  the 
crane  chain,  so  that  the  dies  may  be  turned  into  any  desired 
position.  Water  is  brought  by  the  pipe  D ;  it  is  carried  to  the 
central  gudgeon  pin  A,  and  from  thence,  at  the  back  of  the 
riveter  to  the  cylinder.  The  pipe  leading  from  the  crane  to  D 
consists  of  a  copper  pipe  arranged  spirally  round  the  chain ;  when 
the  riveter  is  lowered  the  spiral  extends ;  when  it  is  raised  the 
spiral  closes ;  so  that  the  water  supply  is  not  affected  by  raising 
or  lowering  the  riveter. 

The  joint  between  the  moving  piston  or  ram,  and  the  cylinder 
walls  is  made  by  a  leather  U-ring,  an  enlarged  view  of  which  is 
given  in  Fig.  88;  with  this  form  of  joint  the  greater  the  pressure 
of  water,  the  more  the  side  of  the  U  -leather  is  forced  against 
the  cylinder  walls.  A  small  ram  placed  at  the  back  of  the 
riveter,  used  for  opening  the  jaws,  is  not  shown.  The  riveter 
arms  are  made  of  cast  steel ;  the  cylinder  is  lined  with  bronze. 

It  will  be  noticed  that  the  cylinder  must  be  curved  at  a 
radius  struck  from  the  gudgeon  pin  A,  in  order  that  the  riveter 
arms  may  move  about  this  centre.  If  the  reader  will  ask  any  of 
his  engineering  friends  how  such  a  cylinder  is  bored,  two  out 
of  three  will  probably  say  that  it  can  only  be  done  by  special 
machinery,  and  it  is  so  stated  in  a  well-known  text-book.  This, 


HYDRAULIC    MACHINERY.  217 

however,  is  not  correct;  the  curved  cylinder  at  the  end  of  a 
riveter  arm  can  be  truly  bored  in  an  ordinary  lathe,  and, 
although  this  book  does  not  deal  with  mechanical  processes  an 
exception  will  be  made  in  the  present  case.  The  method  of 
boring  which  the  author  saw  in  use  some  years  ago  is  as 
follows : — The  boring  tool  is  attached  to  the  lathe  face  plate,  an 
upright  pillar  of  the  size  of  the  gudgeon  pin  A  is  bolted  to  the 
saddle  of  the  lathe,  the  riveter  arm  is  carried  on  this  pillar 
horizontally,  with  its  cylinder  towards  the  tool.  The  riveter 
arm  is  free  to  turn  on  the  pillar,  and  the  latter  is  placed  at  such 
a  distance  from  the  centre  of  the  lathe  that  the  tool  as  it 
revolves  will  take  a  cut  out  of  the  cylinder  if  the  latter  is 
pressed  forward.  A  piece  of  hard  wood,  about  a  couple  of  feet 
long,  is  placed  at  the  back  of  the  cylinder,  and  carried  to  the 
back  centre  of  the  lathe.  The  lathe  is  set  to  work,  the  tool 
revolves,  and  the  turner  feeds  the  cylinder  forward  by  means  of 
the  hand-wheel  on  the  back  centre;  the  arm  carrying  the  cylinder 
being  pivoted  on  the  pillar  referred  to,  the  cylinder  is  bored  to 
the  right  radius. 

Hydraulic  Flanging  Press. — When  a  boiler  plate  to  be 
flanged  is  not  of  too  great  a  diameter  it  is  done  in  a  press  similar 
to  that  shown  by  Fig.  86,  but  a  powerful  press  may  have  two  or 
three  rams  and  cylinders  in  place  of  the  single  ram  shown.  The 
plate  is  made  red  hot  and  is  placed  between  dies,  as  shown  by 
dotted  lines ;  the  plate  is  flanged  at  one  operation  by  the  rising 
of  the  ram. 

In  cases  where  the  plate  to  be  flanged  is  of  irregular  shape 
and  cannot  well  be  done  in  the  manner  described,  the  flanging 
is  done  a  piece  at  a  time  by  means  of  a  press,  as  shown  by 
Fig.  91.  One  ram  holds  the  plate,  while  the  second  ram  turns 
it  over ;  sometimes  there  is  a  third  ram  placed  horizontally  so 
that  it  may  square  up  the  flange  which  has  been  turned  over. 
In  working  with  these  presses  it  is  necessary  to  see  that  the 
plate  is  not  allowed  to  get  too  cold  while  the  operations  are 
being  carried  on,  as  it  has  been  found  that  steel  worked  at  a  blue 
heat  loses  a  large  proportion  of  its  strength. 

Fig.  92  shows  a  hydraulic  punching  machine ;  this  hardly 
needs  a  description  as  its  action  is  the  same  as  that  of  the  fixed 
riveter,  but  without  the  plate-closing  arrangement.  Hydraulic 
shearing  machines  working  upon  the  same  principle  are  also 
used  in  boiler  works  and  ship  yards. 

Hydraulic  Lifts  and  Cranes. — There  are  two  methods  of 
raising  the  cage  of  a  hydraulic  lift.  The  first  is  to  place  it 
directly  on  a  ram,  such  as  is  shown  in  the  illustration  of  the 
hydraulic  press.  This  method  is  simple  and  safe,  but  necessitates  a 


218 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


ram  and  cylinder  of  great  length  ;  it  also  necessitates  a  very  deep 
pit  to  receive  the  cylinder.  The  weight,  too,  of  such  a  ram  is 
somewhat  great  and  a  considerable  amount  of  power  is  expended 
in  raising  it ;  the  weight  of  the  ram  can,  of  course,  be  balanced 
by  counter  weights  and  chains,  but  these  introduce  an  element  of 
risk  and  are  very  undesirable.  A  hydraulic  balancer  somewhat 
similar  to  an  accumulator  is  sometimes  used,  into  which  some  of 
the  water  is  returned  on  the  down  stroke  of  the  ram. 

The  second  method  of  raising  the  cage  of  a  hydraulic  lift  and 
one  which  is  usually  employed  in  hydraulic  cranes,  is  by  means 
of  a  jigger  or  multiplier  as  shown  by  Fig.  93.  By  the  use  of  a 
jigger  a  weight  can  be  lifted  through  a  considerable  distance  with 
a  comparatively  short  ram  and  cylinder.  The  chain  is  passed 
over  multiplying  sheaves,  and  every  sheave  on  the  ram  multiplies 


Fig.  91. — Hydraulic  flanging 
machine. 


Fig.  92. — Hydraulic  punching 
machine. 


the  motion  by  two ;  the  reason  for  this  will  be  seen  by  glancing 
at  the  illustration,  as  for  every  foot  by  which  the  ram  rises  there 
must  be  a  foot  of  chain  on  each  side  of  the  sheave.  Thus  if  there 
are  three  sheaves  and  the  ram  rises  a  foot,  a  motion  of  6  feet  will 
be  given  to  a  weight  at  the  end  of  a  chain.  A  certain  amount 
of  power  is  lost  by  the  friction  of  the  chain  and  pulleys,  but  the 
convenience  of  this  method  of  multiplying  the  lift  outweighs  the 
disadvantage.  Rams  arranged  in  this  way — viz.,  with  the  chain 
passing  over  sheaves — are  used  for  slewing  or  turning  cranes,  as 
well  as  for  raising  the  weight. 

Hydraulic  Jack. — A  hydraulic  jack,  as  shown  by  Fig.  94,  is 
very  useful  for  raising  heavy  weights  when  these  cannot  be  dealt 
with  by  a  crane.  The  principle  upon  which  the  jack  works  is 
the  same  as  that  of  the  press  shown  by  Fig.  86,  but  the  jack  is 


HYDRAULIC    MACHINERY. 


219 


self-contained ;  the  pump  is  placed  inside  the  upper  portion  of 
the  casing;  this  portion  also  holds  the  water  which  the  pump 
plunger  forces  down  into  the  space  A,  and  causes  the  upper 
portion  of  the  jack  to  rise.  The  weight  to  be  raised  is  placed 
either  on  the  top  of  the  jack  or  on  the  projecting  piece  B.  It 
was  by  the  aid  of  such  jacks  that  the  steamer  "Great  Eastern"  was 
successfully  launched  after  having  resisted  all  previous  attempts 


Fig.  93. — Hydraulic  jigger. 


Fig.  94. — Hydraulic  jack. 


to  move  her.     The  Menai  tubular  bridge,  too,  was  raised  into 
position,  a  few  inches  at  a  time,  by  means  of  hydraulic  jacks. 

Hydraulic  Tests. — Parts  of  machinery,  such  as  steam  cylin- 
ders, valves,  boilers,  hydraulic  cylinders,  &c.,  which  are  required 
to  withstand  pressure,  are  usually  tested  hydraulically,  a  hand 
pump  such  as  is  shown  in  Fig.  86  being  used.  In  such  tests  it 
is  necessary  that  the  highest  part  of  the  piece  under  test  should 
be  provided  with  an  air  cock,  and  the  whole  of  the  air  be  ex- 
pelled before  the  full  pressure  is  applied.  If  this  is  not  done, 


220  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

and  the  piece  under  test  fails  to  bear  the  requisite  pressure,  the 
compressed  air  may  cause  broken  pieces  to  fly  in  every  direction, 
possibly  with  serious  results.  Water,  unlike  air,  is  practically 
incompressible,  and  in  the  event  of  the  failure  of  the  piece  under 
hydraulic  test,  a  loud  noise  is  heard,  the  water  escapes,  but  no 
damage  is  done,  provided  no  air  is  present. 

Water  Wheels  and  Turbines. — In  the  days  before  steam 
power  was  available,  mills,  in  which  mechanical  power  was 
required,  were  built  on  the  banks  of  streams  and  rivers,  in 
places  where  a  fall  of  water  could  be  obtained.  If  the  natural 
fall  was  not  sufficiently  great,  it  was  sometimes  increased  by 
placing  a  dam  across  the  stream,  and  thus  raising  the  level  of 
the  water  on  one  side  of  the  dam.  The  water  at  the  higher  level 
then  had  potential  energy,  or  energy  of  position,  and  this  energy 
was  converted  into  mechanical  work  by  means  of  water  wheels. 

Water  wheels  are  seldom  constructed  now,  as  the  capital  which 
has  to  be  expended  in  constructing  the  necessary  masonry  work 
and  wheel  is  usually  greater  than  that  needed  to  purchase  a 
small  engine  and  boiler  capable  of  giving  as  much  power  as  the 
cumbrous  wheel,  and  the  interest  on  the  capital  saved  may  go 
some  way  towards  providing  coal  for  the  steam  plant. 

In  cases  where  an  adequate  supply  of  water  with  a  good  fall 
is  available,  and  considerable  power  is  required,  water  turbines 
are  employed  in  preference  to  water  wheels ;  they  are  much  less 
cumbrous,  require  less  masonry,  are  more  efficient,  and  can  be 
governed  more  accurately  than  the  latter.  The  leading  features 
of  water  wheels  will,  therefore,  only  be  described  briefly  before 
passing  on  to  turbines. 

Water  wheels  are  of  three  kinds — viz.,  Undershot,  Overshot, 
and  Breast  wheels.  Undershot  wheels  are  used  in  cases  where 
the  fall  is  low,  say,  1  to  2  feet,  and  where  a  fair  amount  of  water 
is  running  to  waste.  As  its  name  implies,  the  undershot  wheel 
is  one  in  which  the  water  strikes  the  vanes  of  the  wheel  at  a 
point  below  the  axis,  and  causes  the  wheel  to  rotate.  The 
efficiency  of  an  ordinary  undershot  wheel  is  very  low — viz.,  from 
25  to  30  per  cent. — but  if  the  buckets  are  curved,  as  in  the 
"  Poncelet  ""  wheel,  a  much  higher  efficiency  is  obtained.  The 
Poncelet  undershot  wheel  partakes  somewhat  of  the  nature  of  a 
turbine. 

Overshot  wheels  are  employed  in  cases  where  the  fall  is 
sufficiently  great,  15  feet  or  more,  to  allow  of  the  water  passing 
over  the  top  of  the  wheel  and  descending  on  the  far  side.  In 
this  type  of  wheel  the  vanes  are  shaped  like  buckets,  so  that  the 
weight  of  the  water,  as  well  as  the  energy  with  which  it  enters 
the  buckets,  is  utilised  for  turning  the  wheel.  An  efficiency  of 


HYDRAULIC    MACHINERY.  221 

froin  60  to  75  per  cent,  can  be  obtained  with  this  form  of 
wheel. 

Breast  wheels  are  used  in  cases  where  the  fall,  5  to  15  feet,  is 
hardly  sufficient  to  enable  an  overshot  wheel  to  be  used,  and  where 
the  velocity  of  the  water  is  not  great.  In  the  breast  wheel  the 
water  is  delivered  just  above  the  centre  line  or  axis  of  the  wheel, 
and  its  weight  only  is  used  for  turning  the  wheel.  The  efficiency 
of  an  ordinary  breast  wheel  is  from  30  to  50  per  cent. 

If  the  amount  of  water  flowing  per  minute,  the  distance 
through  which  it  falls,  and  the  efficiency  of  the  water  wheel  are 
known,  it  is  an  easy  matter  to  calculate  the  amount  of  power 
which  can  be  obtained.  An  example  is  given  in  connection  with 
turbines  later. 

Turbines. — The  turbines  chiefly  used  in  Europe  and  in  the 
United  States  of  America  are  of  the  reaction  type,  and  the 
meaning  of  the  words  reaction  turbine  is  as  follows  : — We  have 
read  that,  according  to  Newton's  first  law  of  motion,  "every 
body  continues  in  a  state  of  rest  or  of  uniform  motion  in  a 
straight  line,  except  so  far  as  it  is  compelled  by  force  to  change 
that  state."  Now,  if  a  body  of  water  is  moving  in  a  certain 
direction,  and  is  compelled  to  change  that  direction,  force  is 
required  to  effect  the  change ;  in  the  case  of  a  reaction  turbine, 
the  vanes  of  the  moving  wheel  are  arranged  (see  Fig.  95)  so  as  to 
compel  the  water  to  change  its  direction,  and  the  resistance 
offered  by  the  water  to  this  change  of  direction  exerts  a  re- 
actionary force  on  the  vanes  of  the  moving  turbine  wheel ;  it  is 
this  reactionary  force  which  drives  the  wheel. 

In  the  impulse  form  of  turbine,  the  buckets  of  the  wheel  are 
arranged,  not  so  much  with  a  view  to  change  the  direction  of  the 
jet  of  water,  as  to  oppose  its  progress  and  bring  it  to  a  state  of 
rest.  The  energy  which  the  water  has  acquired  in  falling  from 
a  height — viz.,  kinetic  energy,  or  energy  of  motion,  is  given  up 
to  the  buckets,  and  is  utilised  for  turning  the  wheel. 

Parallel  Flow  or  Axial  Turbine. — Fig.  95  shows  diagram- 
matically  a  turbine  of  this  type,  in  which  the  water  flows  in  a 
direction  parallel  with  the  axis  of  the  turbine.  This  form  of 
turbine  is  generally  known  as  the  Jonval,  from  the  name  of  the 
engineer  who  introduced  it.  The  illustration  is  almost  self- 
explanatory  ;  the  water  descends  through  the  fixed  guide  vanes, 
which  give  it  a  certain  direction ;  the  moving  vanes  prevent  the 
water  from  continuing  in  its  proper  course  and  deflect  it ;  the 
reactionary  force  imparted  to  the  vanes  causes  the  wheel  to 
rotate.  This  form  of  turbine  is  usually  governed  by  throttling 
the  water  as  it  leaves  the  suction  tube  A. 

Radial  Inward  Plow  Turbine. — Fig.  96  shows  a  turbine 


222 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


of  this  type,  in  which  the  water  flows  into  the  turbine  wheel 
in  a  direction  at  right  angles  to  the  axis  of  the  wheel.  This 
form  of  turbine  was  introduced  by  Lord  Kelvin. 

Radial  Outward  Plow  Turbine. — Fig.  97  shows  a  turbine 
of  this  type,  in  which  the  water  flows  in  a  direction  radial  to  the 
axis,  but  outwardly. 


Fig.  95.— Parallel  flow  or  axial  turbine  (Jonval  type). 

In  addition  to  the  three  reaction  turbines  illustrated,  there  is 
the  Mixed  flow  turbine,  which  is  practically  a  combination  of 
those  shown  by  Figs.  95  and  96.  In  this  turbine  the  moving 
blades,  instead  of  ending  as  shown  by  Fig.  96,  are  prolonged  and 


HYDRAULIC    MACHINERY. 


223 


arranged  so  that  they  compel  the  inflowing  water  to  change  its 
course  from  a  horizontal  one  to  a  vertical  one. 

Some  of   the  advantages  and  disadvantages  of   each  type  of 
reaction  turbine  will  now  be  considered. 


Fig.  96. — Radial  inward  flow  turbine. 

The  outward  flow  turbine,  shown  by  Fig.  97,  was  practically 
the  earliest  form  of  reaction  turbine,  and  was  introduced  by  a 
French  engineer  named  Fourneyron.  The  chief  disadvantage  of 
this  turbine  is  the  difficulty  of  governing  it  with  accuracy,  and  a 


224 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


further  disadvantage  is  that  it  must  be  placed  at  the  lowest 
point  of  the  fall,  near  to  the  tail  race.     Turbines  of  this  type 


at 
A.B. 


Fig.  97. — Radial  outward  flow  turbine. 


were  employed  in  the  first  portion  of  what  is  probably  the 
largest  turbine  installation  in  the  world — viz.,  the  Niagara  Falls 
installation.  The  second  instalment  of  turbines  at  this  gene- 


HYDRAULIC    MACHINERY.  225 

rating  station  were,  however,  of  the  inward  flow  type.  This 
installation  will  be  referred  to  again  later.  The  efficiency  of 
the  outward  flow  reaction  turbine  is  from  75  to  80  per  cent. 

The  inward  flow  turbine  has  two  advantages  over  that  of  the 
outward  flow  type,  viz.  : — (1)  Its  variation  of  speed  can  be 
controlled  with  greater  accuracy,  and  (2/  it  may  be  used  with 
a  suction  tube.  The  use  of  this  tube  enables  the  turbine  to  be 
placed  (within  certain  limits  given  later)  near  the  upper  level 
of  the  water,  the  fall  taking  place  in  the  suction  tube.  The 
efficiency  of  this  turbine  is  from  75  to  80  per  cent. 

The  parallel  flow  or  Jonval  turbine  can  also  be  governed  with 
fair  accuracy ;  it  may  be  used  with  a  suction  tube,  and  gives  a 
slightly  higher  efficiency  than  turbines  of  the  radial  flow  type. 
The  efficiency  of  a  good  Jonval  turbine  is  from  80  to  85  per  cent. 
The  Jonval  turbine,  too,  can  be  readily  constructed  for  working 
in  cases  where  the  head  of  water  varies.  In  such  cases  the 
turbine  is  provided  with  two  or  three  rows  of  vanes,  arranged 
concentrically,  instead  of  a  single  row.  When  a  good  head  of 
water  is  available  the  inner  row  is  used,  and  the  outer  rows  are 
shut  off.  When  a  small  head  only  is  available  the  outer  row,  or 
ring  of  vanes,  is  used. 

The  mixed  flow  turbine  may  also  be  used  with  a  suction  tube, 
will  govern  accurately,  and  has  a  still  higher  efficiency  than  a 
Jonval  turbine.  An  efficiency  as  high  as  86  per  cent,  is  often 
obtained  with  a  turbine  of  this  type.  The  mixed  flow  turbine 
is  much  used  in  America ;  it  may  be  of  smaller  dimensions  for  a 
given  power  than  turbines  of  other  types.  The  "  Hercules  "  and 
"Little  Giant"  turbines  are  of  this  type. 

Impulse  Wheels. — These  are  frequently  employed  in  moun- 
tainous countries,  and  are  very  suitable  for  use  in  cases  where 
there  is  a  very  high  fall  of  water,  and  where  a  moderate  amount 
of  power  only  is  required.  The  most  common  form  of  impulse 
turbine  is  the  Pelton  wheel ;  this  consists  merely  of  a  wheel, 
around  the  circumference  of  which  are  a  number  of  buckets.  In 
order  to  avoid  shock  when  the  water  strikes  the  buckets,  these 
are  constructed  in  the  form  of  the  letter  W,  the  upper  part  of 
the  W  points  towards  the  jet,  the  jet  strikes  the  central  portion 
and  glides  down  to  the  bottom  of  the  bucket ;  the  energy  of  the 
water  is  thus  given  up  without  shock.  A  Pelton  wheel  can  be 
governed  with  great  ease  by  throttling  the  jet.  The  largest 
wheel  of  this  type  which  has  been  constructed  gives  about  700 
B.H.P.  The  efficiency  of  a  well-constructed  Pelton  wheel  is  from 
80  to  85  per  cent. 

Before  proceeding  further,  it  may  be  well  to  show  the  student 
how  to  calculate  the  power  which  can  be  obtained  with  any 

15 


226  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

given  quantity  of  water  falling  through  a  given  distance  in  a 
certain  time.  We  have  seen  that  1  H.P.  is  equivalent  to  raising 
33,000  Ibs.  1  foot  high  in  one  minute.  Now,  if  a  body  of  water, 
weighing  33,000  Ibs.,  falls  through  a  distance  of  1  foot,  it  is  also 
the  equivalent  of  1  H.P.,  and  if  there  were  110  loss  in  the 
turbine,  1  H.P.  would  be  obtained  from  this  quantity  of  water 
falling  through  the  distance  mentioned.  Admitting,  however, 
that  there  is  a  loss  in  the  turbine,  and  assuming  that  the 
efficiency  of  the  turbine  is  75  per  cent.,  then,  if  we  multiply 
1  H.P.  by  75,  and  divide  by  100,  we  get  the  actual  horse-power 
-viz.,  -75,  or  |  of  1  H.P. 

The  hydraulic  horse-power  given  by  a  certain  quantity  of 
water  falling  through  a  certain  distance  may,  therefore,  be 
obtained  by  the  following  formula : — 

C  x  62-4  x  F 
33,000 

where  C  =  cubic  feet  of  water  per  minute. 

F  =  fall  in  feet. 
H.P.  =  hydraulic  horse-power. 

If  this  hydraulic  horse-power  is  multiplied  by  the  efficiency  of 
the  turbine  or  water  wheel,  the  actual  or  brake  horse-power 
available  for  external  work  will  be  obtained. 

For  instance,  if  the  hydraulic  horse-power  is  120  H.P.,  and 
the  efficiency  of  the  turbine  is  80  per  cent.,  the  actual  horse- 
power will  be  120  x  80  -=-  100,  or  96  B.H.P.  The  reason  for 
multiplying  C  by  62*4  is  because  a  cubic  foot  of  water  weighs 
approximately  62-4  Ibs.  If  the  amount  of  water  available  is 
given  in  cubic  feet  of  water  per  second,  the  figure  must  be 
multiplied  by  60  to  convert  it  into  cubic  feet  per  minute,  or 
else  the  33,000  Ibs.  must  be  divided  by  60. 

Example. — The  amount  of  water  delivered  to  each  of  the  Niagara  Falls 
turbines  is  430  cubic  feet  per  second,  or  25,800  cubic  feet  per  minute  ;  the 
mean  fall  is  136  feet.  What  horse-power  should  be  obtained?  The  calcu- 
lation is 

25,800  x  62-4  x  136 

—        ^          -  =  6,634  hydraulic  horse-power. 

uo,UUU 

If  we  assume  the  efficiency  of  the  turbines  to  be  75|  per  cent.,  the 
actual  horse-power  available  is  5,010.  The  turbines  are  supposed  to 
give  5,000  B.H.P. 

Suction  Tube. — We  have  said  that  the  use  of  a  suction  tube 
enables  a  turbine  to  be  placed,  within  limits,  near  the  upper 
level  of  the  water.  A  suction  tube,  however,  has  no  useful 
effect  if  it  is  over  30  feet  in  length,  even  if  it  is  of  small  size ; 


HYDRAULIC    MACHINERY.  227 

with  a  suction  tube  of  5  feet  diameter  the  limit  of  useful  length 
is  about  19  or  20  feet,  with  a  tube  10  feet  in  diameter  the  tube 
should  not  be  more  than  10  or  12  feet  long. 

Governing. — The  governing  of  a  turbine  with  sufficient 
accuracy  to  make  it  suitable  for  electric  lighting  purposes  is  not 
altogether  an  easy  task.  If  the  regulation  is  effected  by  means 
of  a  sluice  valve  placed  at  the  outlet  of  the  suction  tube,  such  a 
sluice  valve  requires  to  be  opened  or  closed  to  a  considerable 
extent  before  it  materially  affects  the  quantity  of  water  passing. 
In  some  small  turbines  of  the  Jonval  type  the  governing  is 
effected  by  a  device  which  rolls  and  unrolls  a  scroll  of  leather 
belting  over  the  inlet  vanes,  thus  cutting  a  certain  number  out 
of  use  when  the  turbine  runs  too  fast.  In  the  case  of  the 
Niagara  Falls  turbines  the  governing  is  effected  by  means  of  a 
circular  sluice  running  right  round  the  vanes,  as  shown  by  the 
letter  S  in  Fig.  97.  The  sluice  is  shown  fully  open  •  by  raising 
it  the  whole  outflow  of  water  can  be  stopped.  In  the  Niagara 
turbines  the  vanes  are  divided  into  three  equal  portions  which  are 
successively  closed  as  the  sluice  is  raised.  The  vertical  shaft  of 
each  of  these  turbines  is  provided  with  a  flywheel  14  feet  6  inches 
in  diameter,  weighing  10  tons ;  the  speed  of  the  turbines  is  250 
revs,  per  minute.  The  rim  of  the  flywheel  is  made  of  wrought 
iron,  to  enable  it  to  withstand  the  stress  due  to  centrifugal  force. 
The  reader  may  find  it  interesting  to  work  out  what  the  stress 
in  the  rim  amounts  to,  by  the  rule  given  in  Chapter  vn. 

As  considerable  power  is  required  to  move  the  sluices,  the 
power  is  not  taken  from  a  centrifugal  governor  but  from  the 
turbine  itself;  a  centrifugal  governor  is  used  to  throw  the 
regulating  mechanism  in  and  out  of  gear.  At  Niagara  this 
arrangement  was  not  found  to  act  quickly  enough  to  ensure  very 
good  governing,  and  at  No.  2  power-house  the  sluices  of  the 
turbines  are  worked  by  separate  hydraulic  ram,  the  ram  itself 
being  controlled  by  a  centrifugal  governor.  The  sluices  worked 
in  this  way  can  be  completely  opened  or  closed  in  ten  seconds  : 
the  ordinary  variation  of  speed  does  not  exceed  1  per  cent.,  and 
the  momentary  variation  when  the  whole  load  is  thrown  off  does 
not  exceed  5  per  cent. 

In  some  turbines  the  vanes  themselves  are  opened  and  closed 
in  order  to  regulate  the  speed,  but  this  method  of  governing 
introduces  a  certain  amount  of  complication  which  is  not 
altogether  desirable  in  a  turbine  working  under  water. 

Bearings. — The  bearings  of  turbines  of  small  size  are  often 
made  of  lignum  vitse  and  require  no  lubrication  beyond  that  of  the 
water ;  the  friction,  however,  is  greater  than  with  metal  bearings. 
With  the  latter  it  is  usual  to  force  in  oil  by  means  of  a  force 


228  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

pump.  In  many  cases  an  over-head  bearing  is  used  which  is  not 
in  contact  with  the  water. 

Niagara  Falls  Turbines. — These  turbines,  which  were  the 
largest  in  the  world  when  constructed,  have  many  interesting 
features.  The  turbines,  ten  in  number,  are  designed  to  give 
5,000  H.P.  each,  they  are  placed  in  a  tunnel  at  the  bottom  of  the 
fall,  about  140  feet  below  the  level  of  the  room  in  which  the  electric 
generators  are  fixed.  The  turbines  in  No.  1  power-house  have  a 
double  outflow,  and  are  illustrated  more  or  less  diagrammatically 
by  Fig.  97.  They  are  constructed  so  that  the  pressure  of  the 
water  in  the  turbine  carries  the  great  weight  of  the  vertical 
shaft,  flywheel,  and  rotating  portion  of  the  electric  generator, 
amounting  altogether  to  about  70  tons.  The  lower  internal 
portion  of  the  turbine  casing  is  fixed  and  carries  the  downward 
pressure  due  to  the  column  of  water ;  the  upper  portion  of  the- 
turbine  casing  has  apertures  so  that  the  pressure  of  the  water 
supports  the  upper  moving  wheel  which  carries  the  shaft, 
flywheel,  and  rotating  portion  of  the  dynamo.  The  mean  fall  is 
136  feet,  or,  say,  130  feet,  to  the  top  of  the  turbine,  and  as  every 
foot  of  a  vertical  column  of  water  gives  a  pressure  of  *434  Ib.  pel- 
square  inch,  the  pressure  under  the  moving  wheel  of  the  turbine  is 
about  56  Ibs.  per  square  inch.  The  diameter  of  the  moving  upper- 
portion  of  the  turbine  is  6  feet  3  inches,  the  area  being  4,417 
inches;  multiplying  this  by  56  Ibs.  we  find  that  the  upward 
pressure  of  the  water  is  over  100  tons.  The  shafts  of  these 
turbines  consist  of  hollow  steel  tubes  38  inches  in  diameter  and 
|  inch  thick ;  these  are  lighter  than  solid  shafts.  The  hollow 
shafts  give  place,  however,  to  solid  shafts  10  and  11  inches  in 
diameter  in  the  bearings. 

The  latter  turbines  installed  in  No.  2  power-house  at  Niagara 
are  of  the  inward  flow  type. 

Palls  of  Foyers  Installation. — The  largest  turbine  instal- 
lation in  this  country  is  at  the  Falls  of  Foyers  in  Scotland. 
There  are  five  impulse  turbines  with  vertical  shafts,  each  turbine 
gives  700  B.H.P.  under  a  fall  of  350  feet.  The  turbines  are 
9  feet  in  diameter  and  their  speed  is  140  revs,  per  minute. 

Pumps  for  raising  water  are  usually  of  the  centrifugal  or 
reciprocating  types,  but  there  is  a  third  form  of  pump  of  which 
the  best  known  example  is  the  Pulsometer. 

Centrifugal  Pumps. — A  centrifugal  pump  is  shown  by 
Fig.  98.  This  form  of  pump  is  suitable  for  dealing  with  large 
quantities  of  water  when  the  height  to  which  it  has  to  be  raised,  or 
the  head,  is  not  very  great.  For  heads  up  to  5  or  6  feet,  a  centri- 
fugal pump  is  considered  to  be  more  efficient  than  any  other  type 
of  pump  ;  for  heads  up  to  20  feet,  it  is  considered  to  be  as  efficient 


HYDRAULIC    MACHINERY. 


229 


as  a  good  reciprocating  pump ;  but  when  the  head  is  over  20  feet 
a  centrifugal  pump  is  not  so  efficient  as  a  good  reciprocating 
pump,  but  it  has  other  advantages,  such  as  small  first  cost,  small 
space  occupied,  ability  to  deal  with  muddy  and  gritty  water,  &c. 
A  centrifugal  pump  with  a  single  impeller  will  raise  water  up 
to  60  or  70  feet,  and  by  placing  two  such  pumps  in  series — i.e., 
one  pumping  into  the  other — water  can  be  raised  to  a  height  of 
140  feet  or  more.  •  Pumps  arranged  in  this  way  would  not,  how- 
ever, work  at  the  highest  efficiency,  and  in  cases  where  the  head 
is  greater  than  70  feet  the  pump  is  usually  constructed  with  two 
or  more  impellers  in  one  casing,  so  that  the  passage  from  the 


Fig.  98. — Centrifugal  pump. 


periphery  of  one  impeller  to  the  inlet  of  the  next  is  as  short  and 
direct  as  possible. 

High-speed  multiple-stage  centrifugal  pumps,  driven  either  by 
steam  turbines  or  by  electric  motors,  for  delivering  water  against 
heads  up  to  500  and  600  feet  are  quite  common  on  the  Continent, 
and  it  is  said  that  by  coupling  in  series  two  four-stage  pumps 
Messrs.  Sultzer  have  been  able  to  deliver  water  against  a  head  of 
1,700  feet.  The  pumps  in  question  were  driven  by  three-phase 
motors,  the  speed  of  motors  and  pumps  being  1,040  revs,  per 
minute. 

A  centrifugal  pump,  after  having  been  charged,  will  continue 


230 


MECHANICAL    ENGINEERING   FOR    BEGINNERS. 


to  draw  its  water  from  a  depth  of  25  or  26  feet  below  the  centre 
of  the  pump,  provided  the  joints  of  the  suction  pipe  are  all  tight, 
but  such  a  great  suction  lift  is  not  desirable.  The  suction  lift 
should  not  be  more  than  12  or  15  feet  if  practicable. 

The  efficiency  of  a  well-designed  centrifugal  pump  working 
under  suitable  conditions  is  from  70  to  76  per  cent. 

The  following  table  gives  the  approximate  number  of  gallons 
delivered  per  minute  by  some  centrifugal  pumps  of  Messrs. 
G  Wynnes'  make,  also  the  speeds  at  which  the  juimps  must  be 
driven  to  deliver  water  against  various  heads  : —  . 

TABLE    XIX. 


Size  of  suction"! 

and  discharge  j- 

3" 

4" 

6" 

8" 

10"        12" 

18" 

20" 

pipes,     .         .  j 

Approximated 

number      ofl 

gallons      dis-  V 

184 

325 

730 

1,300 

2,000    2,930 

7,000 

8,500 

charged     per  1 

1 

minute.            J 

Revolutions  per 

minute    for  — 

10-foot  head, 

900 

900 

676 

522 

450 

450 

268 

268 

20-  ,,        „ 

1,160 

1.160 

876 

677 

584 

584 

350 

350 

40 

^t\J~     j  9                9  9 

1,546 

1,546 

1,161 

897 

773 

773 

463 

463 

50-  „        „ 

1,700 

1,700 

1,275 

986 

849 

849 

500 

500 

70-  „       „ 

1,963 

1,963 

1,473 

1.139 

981 

981 

587 

587 

The  speeds  given  in  the  table  are  those  at  which  the  standard 
pumps  will  deliver  the  quantities  of  water  mentioned  at  the  best 
efficiencies,  and  will  give  the  beginner  an  idea  as  to  the  speed  at 
which  such  pumps  must  be  driven,  but  centrifugal  pumps  can  be 
specially  designed  for  lower  or  higher  speeds.  Thus,  if  it  were 
desired  to  couple  a  pump  direct  to  a  vertical  engine,  the  speeds 
given  in  the  table  would  be  too  high,  and  a  pump  having  a  larger 
disc  to  run  at  a  lower  speed  would  be  supplied ;  if  coupled  to  a 
steam  turbine,  the  speed  would  be  too  low,  and  a,  pump  having  a 
smaller  disc  would  be  designed.  There  are,  however,  certain 
limits  in  each  direction ;  if  worked  outside  these,  the  efficiency 
of  the  pump  falls  off  very  rapidly. 

Fig.  98  is  almost  self-explanatory.  All  that  need  be  said  is 
that  the  pump  must  be  charged  with  water  before  starting ;  a 
foot  valve  is  provided  to  prevent  the  charging  water  running 
away,  and  to  avoid  the  necessity  of  recharging  the  pump  every 
time  it  is  stopped  and  started.  The  water  enters  and  leaves 


HYDRAULIC    MACHINERY.  231 

the  pump  as  shown  by  the  arrows ;  the  rotation  of  the  impeller 
or  disc  with  curved  vanes  imparts  the  necessary  motion  to  the 
water. 

The  illustration  shows  a  pump  with  shrouded  disc — i.e.,  one  in 
which  there  is  a  rim  of  metal  on  each  side  of  the  vanes.  This 
shrouding  has  the  effect  of  reducing  the  friction  between  the 
water  which  is  being  rotated  and  the  sides  of  the  pump ;  it  also 
strengthens  the  vanes.  In  unshrouded  or  open  type  disc  pumps, 
the  vanes  are  strengthened  by  central  ribs  or  fins.  In  some 
shrouded  pumps  every  alternate  vane  is  not  carried  right  down 
to  the  boss,  but  ends  at  the  shrouding ;  this  gives  a  freer  entry 
to  the  disc. 

Reciprocating  Pumps. — All  reciprocating  pumps  work  on 
practically  the  same  principle.  The  bucket  or  plunger  draws 
water  into  the  pump  barrel  through  suction  valves  during  one 
stroke,  and  forces  it  out  through  discharge  valves  during  the 
next  stroke.  The  chief  differences  between  reciprocating  pumps 
are  the  manner  in  which  they  are  driven ;  the  form  of  water 
piston — i.e.,  whether  bucket  or  plunger ;  and  the  design  and 
arrangement  of  the  valves.  A  horizontal  reciprocating  pump  of 
the  direct-acting  type  has  been  illustrated  by  Fig.  21,  and  the 
somewhat  uneconomical  method  of  driving  it  referred  to.  A 
vertical  reciprocating  pump,  in  which  the  valves  are  placed  in 
the  bucket,  is  illustrated  by  Fig.  64.  Large  pumps  are  usually 
driven  by  compound  or  triple-expansion  engines,  and  extremely 
economical  results  are  obtained.  A  consumption  of  coal  of 
1-4  Ibs.  per  water  horse-power  is  obtained  in  the  best  pumping 
plants. 

The  water  piston  or  bucket  is  frequently  replaced  by  a  plunger 
or  ram,  which  will  displace  the  desired  quantity  of  water  per 
stroke  without  the  necessity  of  touching  the  sides  of  the  pump 
barrel.  The  plunger  works  through  packing,  and  does  not  get 
cut  or  scored  in  the  same  way  as  a  bucket.  Mine  pumps  and 
other  pumps  which  deal  with  gritty  water  almost  invariably  have 
plungers,  but  in  mines  reciprocating  pumps  are  being  largely 
superseded  by  centrifugal  pumps. 

Pumps  must  be  placed  within  25  feet  of  the  level  of  the  water 
to  be  pumped,  and  if  the  water  is  hot  the  pump  should  be  placed 
below  the  level  of  the  water ;  but  even  when  this  is  done,  there 
is  usually  some  difficulty  in  pumping  very  hot  water.  The  reason 
is  this — We  have  seen  that  the  point  at  which  water  turns  into 
steam  depends  upon  the  pressure  upon  it,  as  well  as  upon  the 
heat  it  contains.  Now,  if  a  pump  attempts  to  lift  water  at  200°  F. 
(and  containing  1,142  B.T.U.),  immediately  the  ram  or  bucket 
moves  quickly  away  from  the  water,  the  latter  tends  to  lag 


232  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

behind  owing  to  inertia  and  to  the  friction  in  the  pipes  and 
valves,  and  thus  a  partial  vacuum  is  created ;  the  water  then, 
instead  of  following  the  ram,  turns  into  steam,  and  the  pump  has 
to  deal  with  steam  instead  of  water.  It  is  for  this  reason  that 
feed  pumps  are  always  arranged  to  force  the  feed  water  through 
an  economiser,  or  an  exhaust  feed- water  heater ;  and  not  to  draw 
the  water  through,  and  then  force  it  into  the  boiler. 

The  successful  working  of  a  pump  depends  very  largely  upon 
the  valves  employed.  The  earliest  form  of  valve,  called  a  clack 
valve,  consisted  of  a  leather  flap ;  the  leather  formed  the  hinge, 
and  the  part  of  the  leather  which  covered  the  opening  was 
strengthened  by  a  metal  plate.  Such  valves  were  only  suitable 
f6r  low  lifts  and  moderate  speeds.  A  clack  valve  is  sometimes 
made  with  a  metal  hinge.  The  pin  is  a  very  loose  fit  in  its  seat, 
so  that  the  face  of  the  valve  may  press  tightly  against  the  valve 
seat.  Such  valves  are  sometimes  used  in  mine  pumps  where  the 
water  is  very  gritty. 

A  plain  gun-metal  disc  valve,  which  rises  off  its  seat  and  is 
brought  back  by  the  pressure  of  water  above  it,  and  by  a  spring, 
is  used  in  hydraulic  pressure  pumps,  but  with  this  form  of  valve 
the  pump  must  run  at  a  low  speed,  and  the  valve  must  have  a 
small  lift ;  otherwise,  a  heavy  blow  is  struck  every  time  the  valve 
closes.  The  duplex  pump  (Fig.  21)  has,  for  the  sake  of  illustra- 
tion, been  shown  with  two  different  types  of  metal  valves,  while 
three  other  types  of  valve  have  been  shown  on  the  drawing  of 
the  air  pump  (Fig.  64). 

.The  head  valves  in  Fig.  64  consist  of  a  grid  and  guard,  the 
valve  itself  being  of  rubber.  The  guard  is  saucer-shaped,  and 
prevents  the  rubber  valve  from  opening  to  too  great  an  extent. 
The  centre  of  the  rubber  valve  does  not  rise  off  its  seat.  This 
type  of  valve  is  often  used  in  air  pumps ;  it  is  unsuitable  for 
cases  where  there  is  a  considerable  head  of  water  above  the 
valve. 

The  bucket  valves  in  the  illustration  consist  of  a  disc  of 
vulcanised  fibre  protected  on  its  back  by  a  bronze  disc.  The 
fibre  valve  rises  off  its  seat,  and  is  closed  by  the  pressure  of 
water  upon  it,  and  by  a  spring.  This  is  a  good  form  of  valve, 
as  the  fibre  does  not  require  to  bend,  and  it  does  not  strike  such 
a  heavy  blow  on  its  seat  as  is  the  case  with  a  solid  metallic 
valve. 

The  foot  valves  shown  are  of  the  Kinghorn  pattern;  this 
valve  consists  of  three  or  more  thin  bronze  discs;  the  discs,  with 
the  exception  of  the  upper  one,  are  perforated,  but  in  such  a 
manner  that  the  perforations  do  not  come  opposite  to  one 
another,  so  that  when  the  discs  are  close  together  no  water 


HYDRAULIC    MACHINERY. 


233 


passes,  but  when  opened  a  slight  distance  the  water  can  pass 
through  the  perforations.  The  lift  of  each  disc  is  very  small, 
and  as  they  are  thin  and  light,  and  as  there  is  a  small  quantity 
of  water  between  each  disc  when  the  plunger  reverses  its  stroke, 


Fig.  99. — Pulsometer  pump. 

the  valve  closes  very  gently.     Kinghorn  valves  are  largely  used 
in  marine  air  pumps,  and  are  found  to  last  a  long  time. 

The  Pulsometer  Pump. — This  pump  which  is  shown  in 
section  by  Fig.  99  is  of  great  simplicity,  and  is  extremely  useful 
in  cases  of  emergency.  The  pump  can  be  lowered  down  a  pit  by 


234  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

a  chain,  steam  being  supplied  to  it  by  a  flexible  or  ordinary 
steam  pipe.  No  foundations  or  fixings  are  required.  The 
pump  consists  of  two  chambers  A,  A,  which  join  at  the  neck  J. 
In  the  neck  is  placed  a  single  ball  valve,  I.  The  ball  oscillates 
on  its  seat,  so  that  while  the  entrance  to  one  chamber  is  open, 
the  entrance  to  the  other  is  closed.  The  pump  is  first  charged 
with  water,  the  steam  enters  one  of  the  chambers  which  happens 
to  be  left  uncovered  by  the  ball,  it  forces  down  the  water  until 
it  reaches  the  outlet  shown  by  dotted  lines,  when  the  surface  of 
the  water  (previously  covered  by  a  film  of  air)  is  broken  up,  the 
steam  then  blows  through  the  outlet  with  a  certain  amount  of 
violence,  comes  into  intimate  contact  with  a  large  amount  of 
cold  water  and  condenses,  a  vacuum  is  formed,  and  the  ball  is 
pulled  over.  The  vacuum  then  causes  water  to  rise  up  through 
the  suction  valves  E,  E,  into  the  chamber  just  emptied;  it  also 
draws  in  a  small  supply  of  air  through  a  snifting  valve  not 
shown  by  the  illustration,  but  placed  high  up  in  the  narrow  part 
of  the  chamber.  The  air  thus  drawn  in  forms  a  cushion  between 
the  steam  and  the  water  on  the  succeeding  stroke.  While  this  has 
been  going  on,  the  operation  first  described  has  been  proceeding 
in  the  second  chamber. 

The  snifting  valves  which  admit  a  small  supply  of  air  at 
every  stroke  are  seldom  if  ever  referred  to  in  published  descrip- 
tions of  the  pump,  or  by  lecturers,  yet  they  are  a  very  important 
feature.  The  pump,  it  is  true,  will  work  without  them,  but  the 
difference  in  the  consumption  of  steam  when  these  valves  are 
removed,  and  their  holes  plugged  with  wood,  is  at  once  notice- 
able. 

The  chamber  B  in  the  illustration  is  merely  an  air-vessel 
connected  to  the  suction,  and  does  not  affect  the  principle  of 
the  pump.  The  air-vessel  is  similar  to  those  often  fitted  to  the 
discharge  of  reciprocating  pumps  for  ensuring  a  fairly  continuous 
flow  of  water ;  at  every  stroke  the  small  quantity  of  air  im- 
prisoned in  the  vessel  is  compressed;  between  strokes  the  air 
expands  and  forces  out  some  of  the  water.  In  the  pulsometer 
the  air-vessel  ensures  a  regular  supply  of  water. 

The  pulsometer  having  no  working  parts  is  not  affected  by 
grit;  it  will  deal  with  semi-liquids  such  as  mud,  liquid  cement, 
sewage-sludge,  &c.  It  requires  no  oil;  the  steam  used  is 
condensed,  and  is  therefore  not  a  nuisance  when  the  pump  is 
worked  down  a  pit  or  in  a  confined  space.  The  pulsometer  will 
raise  water  to  a  height  of  from  70  to  80  feet;  it  should  be  placed 
within  from  6  to  15  feet  of  the  surface  of  the  water,  depending 
upon  the  size  of  the  pump.  The  only  objection  to  this  form  of 
pump  is  that  its  consumption  of  steam  is  higher  than  that  of  a 


HYDRAULIC    MACHINERY.  235 

good  reciprocating  pump.  The  patents  in  connection  with  the 
pulsometer  having  expired,  a  good  many  pumps  on  similar  lines 
are  now  made,  such  as  the  Aqua-thruster,  the  Expulsor  pump, 
and  others.  Whether  these  pumps  are  as  good  as  the  original 
pulsometer  or  not  the  author  is  unable  to  say;  he  has  not  had 
one  under  his  own  observation. 

An  ingenious  device  for  enabling  a  large  quantity  of  water  at 
a  low  level  to  raise  a  small  quantity  to  a  higher  level  is  called  a 
Hydraulic  Ram.  The  action  is  this  :  —  Water  flows  down  a  long 
pipe  and  is  allowed  to  run  to  waste  through  a  valve  which  is 
kept  open  by  a  weak  spring;  when  the  water  has  attained  a 
sufficient  velocity  it  closes  the  valve,  and  forces  open  a  second 
valve,  which  admits  to  the  pipe  leading  to  the  higher  level  ;  the 
kinetic  energy  acquired  by  the  water  is  sufficiently  great  to 
carry  a  small  quantity  to  the  higher  level.  When  the  velocity 
of  the  water  falls  off,  the  waste  valve  again  opens,  and  the  cycle 
is  repeated. 

The  materials  of  which  pumps  should  be  made  for  pumping 
special  liquids  require  to  be  carefully  considered.  The  following 
have  worked  well:  — 

Amoniacal  liquor,        .  .  Cast  iron  entirely. 

Naphtha,    .  .  „  „ 

Tar  and  creosote,         .  .  „  ,, 

Petroleum,  .         .  .  Cast  iron  and  brass. 

Weak  acids,         .         .  .  Gun-metal. 

Sugar,  treacle,  and  malt,  .  ,, 

Vinegar,     ....  Lead. 

Salt  water,  .         .  .  Copper  88,  tin  10,  zinc  2. 

Plow  of  Water  in  Long  Pipes.  —  A  formula  which  gives 
the  head  necessary  to  overcome  the  friction  of  water  in  pipes  is 
sure  to  be  very  useful  to  students.  This  formula  enables  an 
engineer  to  decide  whether  a  pipe  of  a  given  size  is,  or  is  not, 
large  enough  to  pass  a  certain  quantity  of  water  in  a  given  time. 

The  formula,  which  is  given  in  Box's  useful  book  on  hydraulics, 
is  as  follows  :  — 


where  G  =  gallons  per  minute. 

L  =  length  of  pipe  in  yards. 
D  =  diameter  of  pipe  in  inches. 
H  =  head  in  feet. 

As  a  table  of   the   fifth   powers  of   numbers   is   not  always 


236 


MECHANICAL    ENGINEERING   FOR    BEGINNERS. 


available,  and  the  formula  is  such  a  useful  one,  a  table  of  fifth 
powers  is  appended: — 

TABLE   XX. — FIFTH  POWER  OF  NUMBERS. 


No. 

5th  Power. 

No. 

5th  Power. 

No. 

5th  Power. 

1 

1 

34 

45,435,424 

67 

,350,125,107 

2 

32 

35 

52,521,875 

68 

,453,933,568 

3 

243 

36 

60,466,176 

69 

,564,031,349 

4 

1,024 

37 

69,343,957 

70 

,680,700,000 

5 

3,125 

38 

79,235,168 

71 

,804,229,351 

6 

7,776 

39 

90,224,199 

72 

,934,917,632 

7 

16,807 

40 

102,400,000 

73 

2,073,071,593 

8 

32,768 

41 

115,856,201 

74 

2,219,006,624 

9 

59,094 

42 

130,691,232 

75 

2,373,046,875 

10 

100,000 

43 

147,008,443 

76 

2,535,525,376 

11 

161,051 

44 

164,916,222 

77 

2,706,784,157 

12 

248,832 

45 

184,528,125 

78 

2,887,174,368 

13 

371,293 

46 

205,962,976 

79 

3,077,056,399 

14 

537,824 

47 

229,345,007 

80 

3,276,800,000 

15 

759,375 

48 

254,803,968 

81 

3,486,784,401 

16 

1,048,576 

49 

282,475,249 

82 

3,707,398,432 

17 

1,419,857 

50 

312,500,000 

83 

3,939,040,643 

18 

1,889,568 

51 

345,025,251 

84 

4,182,119,424 

19 

2,476,099 

52 

380,204,032 

85 

4,437,053,125 

20 

3,200,000 

53 

418,195,493 

86 

4,704,270,176 

21 

4,084,101 

54 

459,165,024 

87 

4,984,209,207 

22 

5,153,632 

55 

503,284,375 

88 

5,277,319,168 

23 

6,436,343 

56 

550,731,776 

89 

5,584,059,449 

24 

7,962,624 

57 

601,692,057 

90 

5,904,900,000 

25 

9,765,624 

58 

656,356,768 

91 

6,240,321,451 

26 

11,881,376 

59 

714,924,299 

92 

6,590,815,232 

27 

14,348,907 

60 

777,600,000 

93 

6,956,883,693 

28 

17,210,368 

61 

844,596,301 

94 

7,339,040,224 

29 

20,511,149 

62 

916,132,832 

95 

7,737,809,375 

30 

24,300,000 

63 

992,436,543 

96 

8,153,726,976 

31 

28,629,151 

64 

1,073,741,824 

97 

8,587,340,257 

32 

33,554,432 

65 

1,160,290,625 

98 

9,039,207,968 

33 

39,135,393 

66 

1,252,332,576 

99 

9,509,900,499 

Example. — Suppose  we  have  a  pipe  80  yards  long  and  only  6  inches  in 
diameter,  and  we  wish  to  pass  1,000  gallons  of  water  per  minute  through 
it,  what  head  or  pressure  will  be  required  to  make  this  quantity  of  water 
flow  through  the  pipe?  The  calculation  will  be  as  follows  : — 

1,0002  x 


(3  x 


80 
6)5    * 


The  square  of  1,000  is  1,000,000,  and  multiplying  this  by  80  we  get- 
80,000,000 
(18)5 


HYDRAULIC    MACHINERY.  237 


The  fifth  power  of  18  is  1,889,568,  so  we  get 
80,000,000 


The  answer  shows  that  a  head  of  42  feet  would  be  required  to  force 
1,000  gallons  of  water  through  a  pipe  6  inches  in  diameter  and  80  yards 
long.  Such  a  head  would  be  quite  inadmissible  under  ordinary  conditions, 
and  the  calculation  shows,  either  that  the  quantity  of  water  would  have 
to  be  reduced  or  a  larger  pipe  used. 

Now  let  us  try  the  effect  of  passing  the  same  quantity  of  water  per 
minute  through  a  pipe  12  inches  in  diameter. 

1,0002  x  80 
(3  x  12)5  ' 
80,000,000 

-w-' 

80,000,000 

or  ,.   ... . '     _  =  1  -33  feet. 

60,46b,l76 

A  head  of  1  '33  feet,  corresponding  with  a  pressure  of  about  -57  lb.  per 
square  inch,  would  be  a  reasonable  one  to  allow  for  forcing  the  water 
through  the  pipe,  assuming  the  water  has  to  be  pumped  ;  if,  on  the  other 
hand,  a  natural  head  of  between  5  and  6  feet  is  available,  the  reader  will 
find  that  according  to  the  above  formula  a  pipe  9  inches  in  diameter  is 
sufficiently  large. 

USEFUL   HYDRAULIC   MEMORANDA. 

1  cubic  foot  of  fresh  water  at  32°  F.  weighs  62-418  Ibs. 
1  „  „    *39-l°-40°          „         62-425   „ 

1  „  „  60°          ,,         62-321    „ 

1  „  „  100°          „         62-022   „ 

1  „  „  200°          „         60-081    „ 

1  gallon  of  fresh  water  at         60°  „         10  ,, 

1       „        =277-27  cubic  inches. 
1  lb.  of  water  at  32°  measures  27-68  cubic  inches. 
1  „  60°         „         27-72 

A  column  of  water  at  40°  F.  1  foot  high  =  -4335  lb.  per  sq.  in. 
60°  „  =-4328 

A  x  L 

The  capacity  of  a  cylinder  in  gallons  =  977.97  ; 

where  A  =  area  of  cylinder  in  inches. 
L  =  length         „  „ 

Pressure  of  water  at  40°  F.  on  the  side  of  a  vessel 
=  A  x  D  x  62-425  Ibs.; 

where  A  =  area  of  side  in  feet. 
D  =  half  depth  in  feet. 

*  Water  is  at  its  greatest  density  at  a  temperature  of  39'1°. 


239 


CHAPTER  XIII. 
GAS  AND  OIL  ENGINES. 

SUCTION  GAS  PLANT. 

IN  gas  and  oil  engines  the  fuel  is  burnt  in  the  cylinders,  hence 
they  are  called  internal  combustion  engines,  as  distinguished 
from  external  combustion  engines,  such  as  those  driven  by 
steam,  in  which  the  fuel  is  burnt  in  a  furnace  outside  the 
engine. 

Burning  the  fuel  in  the  cylinder  enables  a  higher  thermal 
efficiency  to  be  obtained  than  is  possible  with  an  external  com- 
bustion engine,  but  introduces  difficulties  which  are  absent  in 
the  latter.  The  advantages  and  disadvantages  of  the  gas  engine 
will  be  discussed  later.  In  the  meantime  the  principles  upon 
which  gas  engines  usually  work  will  be  described. 

Otto  Cycle. — In  the  great  majority  of  gas  and  oil  engines 
the  Otto  cycle  or  four-stroke  cycle  is  adopted ;  with  this  cycle 
one  explosion  is  obtained  during  every  two  revolutions  of  the 
engine  crank.  The  action,  commencing  with  the  explosion,  is  as 
follows  : — 

.  1.  Explosion — Outward  stroke. 

2.  Inward  stroke — Piston  drives  out  exhaust  gases. 

3.  Outward  stroke — Piston  draws  in  mixture  of  gas  and  air. 

4.  Inward  stroke — Piston  compresses  mixture. 

When  the  cycle  is  again  repeated. 

Description  of  Engine. — Fig.  100  represents  a  10  B.H.P. 
gas  engine  *  stripped  of  all  details,  such  as  governor,  lubricators, 
cams,  levers,  &c.  The  piston  is  shown  at  the  beginning  of  its 
stroke,  having  compressed  its  charge  ready  for  ignition.  After 
the  explosion  has  taken  place,  and  the  piston  has  reached  the 
end  of  its  stroke,  the  exhaust  valve  will  be  lifted  by  a  lever 
worked  from  a  cam  on  the  shaft,  a  portion  of  which  is  shown 
at  the  front  end  of  the  engine.  This  shaft  is  driven  by  gearing, 
and  only  makes  one  revolution  for  every  two  of  the  engine. 
After  the  burnt  gases  have  been  driven  out,  the  gas  and  air 
valves,  which  work  horizontally,  are  opened  by  levers  worked 
from  cams  on  the  shaft  already  referred  to,  and  a  charge  of  gas 

*  Constructed  by  the  Railway  and  General  Engineering  Company  of 
Nottingham. 


240 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


and  air  is  drawn  in ;  this  mixture  is  then  compressed  by  the 
piston  on  its  return  stroke,  when  all  is  ready  for  ignition. 

The  engine  illustrated  has  many  good  features.     In  the  first 
place,  the  valves  and  their  seatiiigs  can  easily  be  removed  for 


examination  and  cleaning  ;  in  the  second  place,  the  cylinder  and 
water-jacket  are  not  overhung  and  carried  entirely  by  the  flange 
at  one  end,  as  is  so  commonly  met  with.  It  may  be  considered 
by  some  that  valves  working  horizontally,  as  the  gas  and  air 


GAS   AND   OIL    ENGINES.  241 

valves  in  the  engine  illustrated,  are  not  so  good  as  those  which 
have  a  vertical  lift;  in  practice,  however,  the  horizontal  valves 
have  been  found  to  work  well. 

The  points  about  a  gas  engine  to  which  attention  may  be 
directed,  taking  them  in  the  order  of  the  cycle  already  given, 
are  as  follows  : — 

Methods  of  Ignition. — In  the  earlier  Otto  engines,  in  which 
a  single  slide  valve  was  used  for  admitting  the  mixture  and 
exhausting  the  burnt  gas,  the  slide  valve  brought  forward  at  the 
right  moment  a  small  pocketful  of  lighted  gas,  and  placed  it 
opposite  a  port  at  the  end  of  the  cylinder,  the  result  being  that 
the  compressed  mixture  immediately  exploded.  The  slide  valve 
with  its  rubbing  surfaces  was  found  to  be  unsuitable  for  high 
speeds  and  for  high  temperatures  and  pressures,  and  was  aban- 
doned in  favour  of  valves  of  the  mushroom  type,  as  shown  by 
the  illustration.  When  the  mushroom  valves  were  adopted,  the 
tube  method  of  ignition  came  into  general  use.  This  method 
consists  of  keeping  a  tube  red  hot  by  allowing  a  flame  to  play 
constantly  upon  its  outside ;  when  the  mixture  has  been 
compressed,  a  small  quantity  is  admitted  to  the  inside  of  the 
tube,  and  ignition  takes  place.  This  method  of  ignition  is  still 
largely  used,  but  in  large  gas  engines,  and  in  many  modern 
small  gas  and  oil  engines,  the  electric  method  of  ignition  is 
employed. 

Electric  ignition  may  be  carried  out  in  several  ways.  In 
the  majority  of  cases  a  small  dynamo,  called  a  magneto,  is  used. 
If  the  current  produced  by  the  magneto  is  of  low  tension — the 
type  usually  employed  with  gas  engines — then  the  current  is 
suddenly  interrupted  in  a  place  which  is  accessible  to  the 
explosive  mixture ;  this  sudden  breaking  of  the  circuit  causes 
a  spark  which  fires  the  mixture.  If  the  magneto  is  of  the  high- 
tension  type,  as  frequently  used  with  petrol  engines,  the  circuit 
is  made  and  broken  outside  the  cylinder,  the  spark  inside  the 
cylinder  or  explosion  chamber  being  caused  by  the  current 
jumping  across  the  fixed  points  of  a  sparking  plug  screwed 
into  the  cylinder. 

When  a  magneto  is  not  used,  a  2-cell  accumulator  giving  a 
current  at  an  E.M.F.  of  about  4  volts  is  generally  employed, 
and  an  induction  coil  is  used  to  increase  the  E.M.F.  of  the 
current,  so  as  to  enable  it  to  jump  across  the  points  of  the 
sparking  plug.  The  current  in  such  cases  is  interrupted  by 
means  of  a  trembler  placed  at  the  top  of  the  induction  coil. 
Such  accumulators  require  to  be  charged  periodically,  and  are 
much  more  troublesome  than  a  magneto. 

In  the  Diesel  oil  engine  no  ignition  apparatus  of  any  kind  is 

16 


242  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

used;  the  air  is  compressed  to  a  pressure  of  about  500  Ibs.  per 
square  inch,  corresponding  with  a  temperature  of  about  1,000° 
F.,  which  temperature  is  sufficiently  high  to  fire  the  fuel 
immediately  it  is  injected. 

Exhausting  and  Scavenging. — The  mere  fact  of  opening  the 
exhaust  valve  is  not  sufficient  to  ensure  that  all  the  burnt  gases 
will  leave  the  cylinder;  in  fact,  the  clearances  will  remain  filled 
with  them.  These  remaining  products  of  combustion  reduce 
the  value  of  the  next  incoming  explosive  mixture,  and,  if  any 
incandescent  particles  are  left  in  the  cylinder,  they  may  cause 
pre-ignition  of  the  charge.  To  ensure  the  removal  of  the  burnt 
gases  various  expedients,  called  scavenging,  are  adopted.  The 
simplest  is  that  usually  employed  in  connection  with  the 
Crossley  engine ;  it  consists  of  an  exhaust  pipe  of  suitable 
diameter  60  or  65  feet  long.  The  exhaust  gases  travel  along  this 
pipe  at  such  a  speed  that  (to  express  it  colloquially)  they  find  it 
difficult  to  stop,  and  actually  create  a  vacuum  in  the  cylinder. 

This  arrangement  is  not  altogether  satisfactory  at  light  loads, 
and  in  large  gas  engines  it  is  not  practicable  to  arrange  for  a 
suitable  length  and  diameter  of  exhaust  pipe.  Air  pumps  are 
therefore  frequently  used  for  forcing  air  into  the  cylinders  and 
thus  scavenging  them  before  the  fresh  charge  is  admitted.  The 
air  pumps  are  driven  by  some  reciprocating  part  of  the  engine  or 
by  an  auxiliary  crank. 

Drawing  in  Mixture  of  Gas  and  Air  and  Governing. — A 
gas  engine  may  be  governed  in  three  ways — (1)  By  omitting  to 
open  the  gas  inlet  valve  when  the  speed  of  the  engine  rises 
beyond  a  certain  point ;  this  is  called  governing  on  the  hit-and- 
miss  principle,  the  miss  occurring  when  the  speed  is  too  high.  (2) 
By  altering  the  mixture  of  gas  and  air.  (3)  By  altering  the 
quantity  of  the  mixture  admitted  without  altering  the  quality. 
The  first  method  is  that  usually  adopted  in  small  and  medium 
powered  gas  engines  ;  it  is  simple,  but  the  governing  is  not 
very  accurate  unless  a  really  heavy  flywheel  is  used.  Governing 
by  either  the  second  or  third  methods  is  preferable — viz.,  by 
altering  the  quality  or  quantity  of  the  mixture.  By  reducing 
the  quantity  of  the  mixture  admitted  to  the  cylinder,  the 
compression  is  reduced  and  the  full  value  of  the  explosive 
mixture  is  not  obtained.  On  the  other  hand,  by  reducing  the 
amount  of  gas  admitted  and  so  altering  the  quality  of  the 
mixture,  the  gas  is  not  burnt  to  the  best  advantage.  Both 
systems  have  their  advocates. 

Compressing  the  Mixture. — It  has  been  conclusively  proved 
that  a  high  degree  of  compression  results  in  a  high  explosive 
force,  and  vice  versa;  a  mixture  which  is  highly  compressed 


GAS    AND    OIL    ENGINES. 


243 


burns  more  quickly  than  one  which  is  only  slightly  compressed. 
Consequently,  during  recent  years  the  clearances  in  gas  engines 
have  been  reduced,  and  the  compression  increased.  The  amount 
to  which  the  charge  can  safely  be  compressed  without  risk  of 
pre-ignition,  depends  upon  the  scavenging  and  water-cooling 
arrangements. 

In  modern  gas  engines  of  large  power,  and  even  in  small 
engines  in  which  economy  is  studied,  the  compression  ranges 
from  150  to  200  Ibs.  per  square  inch.  In  the  Diesel  engine,  the 
air  is  compressed  to  about  500  Ibs. ;  the  fuel  is  then  injected  by 
a  blast  of  air  at  a  pressure  of  about  600  to  650  Ibs.,  when  the 
mixture  immediately  ignites. 

We  have  said  that  the  majority  of  gas  engines  work  on  the 


Exhaust  Ports. 
Fig.  101. — Korting  gas  engine. 

Otto  cycle ;  the  most  noteworthy  exception  is  the  Korting  engine, 
shown  by  Fig.  101.  This  engine  is  double-acting,  and  there  are 
no  exhaust  valves.  The  exhaust  ports  are  placed  in  the  middle 
of  the  cylinder,  as  shown  by  dotted  lines ;  when  the  piston 
reaches  the  end  of  its  stroke  in  either  direction,  it  uncovers  the 
exhaust  ports ;  as  soon  as  the  ports  are  uncovered,  a  charge  of 
fresh  air  is  blown  in,  and  the  burnt  gases  are  expelled.  When 
the  piston  has  moved  sufficiently  far  to  close  the  exhaust  ports, 
a  supply  of  gas  is  pumped  in,  the  air  and  gas  inlet  valve  then 
closes,  and  the  mixture  is  compressed. 

By  this  arrangement  there  is  an  impulse  every  outward  stroke, 
and  as  the  engine  is  double-acting,  there  are  two  impulses  per 
revolution.  Such  an  engine  occupies  much  less  space  than  a 
single-cylinder  engine,  in  which  there  is  only  one  impulse  during 


244  MECHANICAL   ENGINEERING   FOB    BEGINNERS. 

every  two  revolutions.  The  gas  and  air  pumps  are  placed  along- 
side the  cylinder,  and  are  worked  off  an  auxiliary  crank ;  they 
are  not  shown  in  the  illustration. 

Double-acting  engines,  working  on  the  four-stroke  cycle,  are 
frequently  made  on  the  Continent.  These  engines  occupy  less 
space  than  those  of  the  single-acting  type,  but  the  parts  are  not 
quite  so  accessible  for  cleaning ;  moreover,  a  gland  is  required  at 
the  piston-rod  end  of  the  cylinder.  It  will  be  noticed  that  in  the 
single-acting  engine,  illustrated  by  Fig.  100,  one  side  of  the  piston 
or  trunk  is  open  to  the  air,  and  no  gland  is  required ;  the  piston 
can  be  withdrawn  by  undoing  the  bolts  of  the  connecting-rod 
brasses.  These  are  good  features.  The  gland  of  a  double-acting 
gas  engine,  unless  designed  with  great  care,  and  water-cooled,  is 
likely  to  be  a  source  of  trouble. 

In  double-acting  engines,  and  in  single-acting  engines  when 
the  power  exceeds  250  B.H.P.,  it  is  advisable  to  water-cool  the 
piston  and  rod ;  water  for  this  purpose  is  admitted  to  the  cross- 
head  by  means  of  swinging  link  pipes,  and  thence  through  the 
hollow  piston-rod  to  the  piston. 

Amount  of  Gas  Consumed. — The  amount  of  gas  consumed 
in  an  engine  per  horse-power  developed,  depends  largely  upon 
the  richness  or  otherwise  of  the  gas.  The  number  of  British 
thermal  units  contained  in  gases  made  by  different  processes  is 
approximately  as  follows  : — 

Town  gas,       .         .         .600  to  680  B.T.U.  per  cubic  foot. 
Dowson  gas,  .         .150  to  164       „  „ 

Producer  gas  (suction  gas  ] 

plant),         .         .         .  V 125  to  155       „  „ 

Mond  gas,      .         .         .  ) 
Blast-furnace  gas,  .         .     100  to  120       „ 

Town  gas  requires  to  be  mixed  with  about  10  times  its  volume 
of  air,  while  producer  gas  requires  about  1J  or  1J  times  its 
volume  of  air,  in  order  to  make  the  best  explosive  mixture  in 
the  engine.  The  consumption  of  rich  town  gas  in  an  engine  of, 
say,  20  H.P.  should  be  between  14  and  16  cubic  feet  per 
I.H.P.  per  hour,  which  is  equivalent  to  between  16J  and 
19  cubic  feet  per  B.H.P.  The  consumption  of  producer  gas 
containing  130  or  140  B.T.U.  in  a  good  engine  should  not 
exceed  about  60  cubic  feet  per  I.H.P.  per  hour,  or  70  cubic 
feet  per  B.H.P. 

Horse-power  of  Gas  Engines. — The  indicated  horse-power 
of  a  gas  engine,  working  on  the  Otto  or  any  other  cycle,  may 
be  found,  provided  the  mean  pressure  in  the  cylinder  is  known, 
by  the  following  formula  : — 


GAS   AND    OIL    ENGINES.  245 

S    X   N    x   A   x    P. 

33,000 

where   S  =  stroke  of  the  piston  in  feet. 

N  =  number  of  explosions  per  minute. 

A  =  area  of  the  piston  in  inches. 

P  =  mean  pressure  exerted  on  the  piston  during  the  stroke. 

Example. — What  is  the  indicated  horse-power  of  a  gas  engine  having  a 
cylinder  10  inches  in  diameter,  18  inches  stroke,  and  speed  190  revs,  per 
minute,  the  mean  effective  pressure  on  the  piston  bemg  70  Ibs.  ?  The 
calculation  is 

1-5  x  95  x  78-5  x  70 
33,000 

The  above  calculation  is  for  an  engine  working  on  the  Otto  cycle,  in 
which  there  is  an  explosion  for  every  2  revs,  of  the  engine. 

The  actual  horse-power  of  a  gas  engine  can,  however,  only  be 
determined  with  accuracy  by  a  brake  trial,  or  by  coupling  it  to  a 
dynamo,  the  efficiency  of  which  is  known,  for,  in  a  gas  engine, 
the  indicator  cards  which  purport  to  give  the  mean  pressure  are 
not  altogether  to  be  relied  upon. 

The  maximum  temperature  attained  in  the  interior  of  a  gas- 
engine  cylinder  by  using  a  mixture  of  1  part  of  town  gas  to  9 
parts  of  air,  and  compressing  it  to  about  100  Ibs.  pressure,  is 
between  3,000°  and  3,500°  F.  The  higher  of  these  temperatures 
is  above  the  melting  point  of  platinum,  so  that  the  necessity  of 
a  water-jacket  is  apparent.  The  maximum  pressure  reached  in 
the  cylinder  is  about  3  or  3J  times  the  compression  pressure. 

Thermal  Efficiency.  —  The  thermal  efficiency,  either  per 
indicated  or  per  brake  horse-power,  of  a  gas  engine  is  easily 
ascertained  if  the  number  of  cubic  feet  of  gas  used  per  horse- 
power per  hour,  and  the  calorific  value  of  the  gas,  are  known. 
Let  us  find  the  thermal  efficiency  per  B.H.P.  of  a  large  gas 
engine  using  60  cubic  feet  of  producer  gas  per  I.H.P.  per  hour. 
If  the  mechanical  efficiency  of  the  engine  is  between  85J  and  86 
per  cent.,  the  consumption  of  gas  will  be  70  cubic  feet  per  B.H.P. 
per  hour.  We  will  assume  that  the  calorific  value  of  the  gas  is 
130  B.T.U.  per  cubic  foot.  The  heat  used  by  the  engine  per 
B.H.P.  is,  therefore,  70  x  130  =  9,100  B.T.U.  per  hour.  We 
read  in  Chapter  in.  that  a  B.T.U.  =  772  ft.-lbs.,  and  a  H.P. 
33,000  ft.-lbs;  therefore,  42-75  B.T.U.  per  minute,  or  2,565 
B.T.U.  per  hour,  are  the  equivalent  of  1  H.P.  The  engine  uses 
9,100  B.T.U.  per  brake  horse-power,  so  that  the  thermal  effi- 
ciency of  the  engine  is  *  '  =  -28.  The  absolute  thermal 

y,  luu 

efficiency  of  the  engine  per  B.H.P.  is  therefore  '28. 


246 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


If  the  engine  only  uses  *28  of  the  heat  supplied  to  it,  the 
question  which  naturally  arises  is  —  What  becomes  of  the 
remainder  of  the  heat  1  The  distribution  of  the  heat  originally 
contained  in  the  gas  is  approximately  as  follows  : — 

•38  carried  off  in  the  exhaust  gases. 

•29  carried  off  by  the  jacket  cooling  water  by  radiation. 

•28  given  up  as  brake  horse-power. 

•05  used  in  overcoming  the  friction  of  the  engine. 


1-00 

Although  a  thermal  efficiency  of  '28  is  apparently  low,  it 
compares  favourably  with  that  of  a  steam  engine.  The  absolute 
thermal  efficiency  of  a  triple-expansion  condensing  engine, 
working  with  saturated  steam  at  160  Ibs.  pressure,  and  using 
14  Ibs.  of  steam  per  B.H.P.,  is  only  -162. 

Speeds  of  Gas  Engines. — The  speeds  at  which  gas  engines 
usually  run  are  approximately  as  follows  : — 

5  B.H.P.,  250  revs,  per  minute. 

10  220 

20  200 

50  175 

100  160 

500  120 

Suction  Gas  Plant. — The  history  of  the  modern  suction 
plant  is  briefly  as  follows : — It  was  found  by  the  late  Mr.  Siemens 
that  it  was  more  economical  and  convenient  to  turn  cheap  fuel 
into  gas,  and  then  burn  the  gas  in  regenerative  furnaces,  than  to 
utilise  the  heat  derivable  from  complete  combustion  of  the  coal 
in  the  first  place.  To  turn  the  coal  into  gas  it  was  placed  on  a 
deep  layer  on  a  grate  and  partially  burnt,  the  air  that  found  its 
way  through  the  coal  not  being  of  sufficient  quantity  to  allow  of 
complete  combustion.  The  gas  that  was  given  off  consisted 
chiefly  of  carbon  monoxide  (CO)  and  nitrogen,  together  with  a 
small  quantity  of  hydrogen.  The  gas  was  of  low  calorific  value, 
and,  in  its  production,  not  less  than  30  per  cent,  of  the  heat 
contained  in  the  coal  was  lost.  The  small  quantity  of  hydrogen, 
too,  which  it  contained  prevented  the  rapid  ignition  which  is 
necessary  in  an  explosive  mixture.  In  order  to  improve  the 
quality  of  the  gas,  Mr.  Dowson,  in  1878,  turned  a  jet  of  steam 
(H2O)  into  the  hot  fuel,  the  result  being  that  the  hydrogen  was 
liberated  and  was  added  to  the  gas,  while  the  oxygen  combined 
with  the  carbon.  The  temperature  of  the  furnace  was  reduced 


GAS   AND    OIL    ENGINES.  247 

and  the  quality  of  the  gas  improved,  the  calorific  value  of  the 
Dowson  gas  being  within  15  per  cent,  of  that  of  the  solid  coal. 

Although  gas  produced  on  the  Dowson  system  was  of  fair 
calorific  value  and  inexpensive,  there  were  at  least  two  reasons 
why  the  system  did  not  come  into  general  use  for  driving  gas 
engines.  The  first  was  the  high  cost  of  the  plant,  and  the 
second  the  great  amount  of  floor  space  required.  A  boiler, 
working  at  about  50  Ibs.  pressure,  was  required  to  drive  the 
steam  and  air  through  the  coal,  and  a  gas-holder  was  provided 
to  contain  the  gas  after  it  had  been  produced. 

In  a  modern  suction  gas  plant  the  heat  generated  in  the 
producer  is  used  to  form  the  steam,  and,  as  the  heat  is  not 
sufficiently  great  to  generate  steam  at  a  pressure  sufficiently 
high  to  force  it  through  the  fuel,  the  outward  stroke  of  the 
engine  is  used  to  suck  the  steam  and  air  through  the  fire.  A 
gas-holder,  if  placed  between  the  generator  and  the  engine, 
would  do  harm,  as  the  effects  of  the  suction  stroke  would  not  be 
felt  in  the  generator :  hence  both  boiler  and  gas-holder  have  been 
done  away  with. 

One  of  the  simplest  forms  of  suction  gas  plants*  now  made  is 
shown  by  Fig.  102.  The  generator  or  producer  A  in  which  the 
fuel  is  burnt  is  surrounded  by  a  jacket  or  vaporiser,  B;  this 
jacket  is  filled  with  coke  or  any  similar  material;  the  material  is 
kept  moist  by  sprinkling  water  upon  it,  and  is  warmed  by  the 
heat  of  the  producer.  The  air  required  for  combustion  is  com- 
pelled to  pass  through  the  belt  of  wet  coke  in  the  jacket,  and  in 
doing  so  becomes  charged  with  moisture;  the  suction  stroke  of 
the  engine  draws  the  hot  moist  air  through  the  fuel,  as  already 
explained.  The  amount  of  water  sprinkled  on  to  the  coke  in  the 
jacket  is  regulated  automatically  by  the  suction  of  the  engine. 
After  the  gas  has  been  made  in  the  producer,  it  is  necessary  to 
pass  it  through  the  scrubber  shown  on  the  right-hand  side  of  the 
illustration.  The  scrubber  consists  merely  of  a  cylinder  filled 
with  coke,  through  which  a  stream  of  water  is  kept  flowing  in  a 
downward  direction ;  this  water  carries  away  any  tar  suspended 
in  the  gas.  After  leaving  the  scrubber  the  gas  is  usually  taken 
to  a  box,  which  helps  to  reduce  any  violent  fluctuations  of 
pressure,  and  is  then  taken  to  the  engine.  The  fan  shown  on 
the  left  side  of  the  illustration  is  only  used  at  starting,  when  the 
cock  C  is  opened,  and  the  gases  pass  away  to  the  atmosphere. 

Amount  of  Gas  made  and  Power  developed  per  Pound 

of  Coal. — A  producer  similar  to  the  one  illustrated  will  make 

about  80  cubic  feet  of  gas  for  every  pound  of  anthracite  coal 

consumed,  and  as  a  good  engine  of  fairly  large  size  uses  only 

*  Constructed  by  the  Dowson  Gas  Co.  of  Westminster. 


248 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


about  70  cubic  feet  of  producer  gas  per  B.H.P.,  it  follows  that 
1   Ib.   of   anthracite   will   give   about    M4   B.H.P.      It   is  the 


Fig.  102. — Suction  gas  producer. 

practice  of   many  makers    of   suction   gas   plants  to   guarantee 
that    -9  Ib.   of   anthracite    will   give    1    B.H.P.,    but   at   some 


GAS   AND    OIL    ENGINES. 


249 


trials  of  several  15  to  20  B.H.P.  engines  working  with  suction 
gas,  carried  out  by  the  Royal  Agricultural  Society  in  1906,  the 
consumption  was  a  little  higher.  The  lowest  consumption  was 
1-04  Ibs.  of  anthracite,  and  the  highest  147  Ibs.,  the  average 
consumption  of  the  eleven  engines  being  1-21  Ibs.  of  anthracite 
per  B.H.P.  The  consumption  of  an  engine  working  with  a 
Dowson  producer,  as  illustrated,  was,  at  the  trials  in  question, 
1*09  Ibs.  per  B.H.P.  The  average  consumption  of  eleven  plants 
working  with  coke  was  1*4  Ibs.  per  B.H.P. 

Water  required  for  Suction  Gas  Plants. — It  is  usually 
reckoned  that  about  1  gallon  of  water  is  required  per  B.H.P., 
nine-tenths  of  which  is  used  in  the  scrubber,  the  remaining 
tenth  in  the  generator.  In  the  trials  carried  out  by  the  Royal 
Agricultural  Society,  the  consumption  of  water  was  considerably 
higher  than  1  gallon.  The  average  consumption  of  twelve 
plants  was  1-89  gallons  of  water  per  B.H.P.,  or  about  three- 
fourths  as  much  as  would  be  used  by  a  good  non-condensing 
steam  engine.  The  amount  of  water  used  by  the  plant  which 
secured  the  gold  medal  was  1'14  gallons  per  B.H.P. 

Mond  Gas. — Bituminous  coal  cannot  be  used  in  a  suction 
gas  producer,  only  anthracite  coal  or  coke.  Dr.  Mond,  however, 
has  introduced  a  process  by  which  cheap  bituminous  coal  may 
be  used  for  the  production  of  gas.  The  Mond  process  consists  of 
introducing  an  enormous  quantity  of  superheated  steam  and  air 
to  the  generator.  The  greater  portion  of  the  steam  passes  out 
of  the  producer  undecomposed ;  it  is  then  condensed,  and  its 
heat  is  utilised.  The  temperature  of  the  generator  is  kept  very 
low  by  the  introduction  of  the  very  large  quantity  of  steam  and 
air,  and  a  large  amount  of  sulphate  of  ammonia  is  obtained  ;  this 
forms  a  valuable  by-product.  No  tar  is  produced.  This  system, 
like  that  first  introduced  by  Mr.  Dowson,  requires  a  separate 
boiler.  About  70  cubic  feet  of  gas  are  produced  from  1  Ib.  of 
bituminous  coal. 

The  analyses  of  gases  produced  by  the   Dowson,  Suction,  and   Mond 
processes  are  approximately  as  follows : — 


Dowson 

System. 

Suction  Gas 
System. 

Mond 
System. 

Hydrogen  (H),  . 
Oxygen  (0), 
Carbon  monoxide  (CO), 
,,      dioxide  (C02), 
Marsh  gas  (CH4), 
Nitrogen  (N),    . 

19-8 

23:8 
6-3 
1-3 

48-8 

17-5  to  20-5 
0-5          0-2 
18-5        21-5 
7-0         7'5 
1-5            -5 
55-0       49-8 

24-8 

13-2 
12-9 
2-3 

46-8 

100-0 

100-0      100-0 

100-0 

250  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

Starting  Gas  Engines. — Small  gas  engines  may  be  started 
by  the  driver  pulling  round  the  flywheel  by  hand,  and  thus 
drawing  in  the  explosive  mixture  and  compressing  it ;  but  in 
the  case  of  large  engines  this  is  impracticable,  and  special  means 
for  starting  have  to  be  provided.  Some  engines,  such  as  the 
Westinghouse  gas  engine  and  the  Diesel  oil  engine,  are  started 
by  means  of  compressed  air;  the  engine  is  provided  with  a  pump 
which  compresses  a  certain  quantity  of  air  before  stopping,  and 
forces  it  into  a  drum  provided  for  the  purpose.  Other  engines 
are  provided  with  a  starter,  consisting  of  a  chamber  into  which 
a  mixture  of  gas  and  air  is  pumped  by  hand;  the  flywheel  of  the 
engine  is  then  barred  round  by  a  lever  with  a  pawl  at  the  end, 
teeth  being  cast  in  the  flywheel  for  the  purpose.  When  the 
piston  is  in  the  right  position,  the  cylinder  is  placed  in  com- 
munication with  the  starter,  the  mixture  is  fired,  and  the 
resulting  explosion  in  the  cylinder  is  sufficient  to  start  the 
engine.  Other  engines  merely  have  a  pinion  with  large  hand 
wheel ;  the  pinion  works  in  mesh  with  teeth  in  the  large  fly- 
wheel ;  the  large  hand-wheel  is  constructed  on  the  free-wheel 
principle,  so  that  it  is  not  driven  round  when  the  gas  engine 
is  running. 

Comparative  Merits  of  Steam,  Gas,  and  Oil  Engines. — 
The  cost  of  fuel  per  horse-power  for  gas,  steam,  oil,  and  petrol 
engines  is  approximately  as  follows  : — 

Penny  per  B.H.P. 
per  hour. 

Gas  engine  using  producer  gas,  and  consuming 
1J  Ibs.  of  coke  per  B.H.P.  The  cost  of  coke 
is  taken  at  15s.  per  ton,  '1 

Gas  engine  using  20  cubic  feet  of  town  gas  per 
B.H.P.  The  cost  of  town  gas  is  taken  at 
2s.  Cd.  per  1,000  feet,  .  -6 

Condensing  steam  engine  using  1J  Ibs.  of  coal 
per  B.H.P.  The  cost  of  coal  is  taken  at  15s. 
per  ton,  .......  "14 

Non-condensing  steam  engine  using  3-75  Ibs.  of 
coal  per  B.H.P.  The  cost  of  coal  is  taken  at 
15s.  per  ton, -3 

Oil  engine  using  '085  of  a  gallon,  at  4d.  per 

gallon,  ........  "34 

Petrol  engine  using  "11  of  a  gallon,  at  Is.  4d. 

per  gallon,  .  .  .  .  .  .  .1-76 

The  cost  of  coal  per  ton,  of  gas  per  1,000  cubic  feet,  and  of  oil 
and  petrol  per  gallon  are  given  so  that  the  reader  can  make  any 


GAS    AND    OIL    ENGINES.  251 

necessary  correction  in  the  comparative  costs  to  suit  the  cost  of 
fuel  in  his  own  locality. 

If  the  water  required  for  either  steam  or  gas  plant  has  to  be 
paid  for,  the  cost  must  be  added  to  the  figures  given. 

It  will  be  seen  that,  so  far  as  fuel  alone  is  concerned,  a  gas 
engine  using  producer  gas  is  the  cheapest  form  of  motive  power, 
also  that  a  gas  engine  using  town  gas  at  2s.  6d.  per  1,000  feet  is, 
with  the  exception  of  a  petrol  engine,  the  most  expensive  of 
those  given.  Fuel,  however,  is  not  the  only  item  to  be  taken 
into  consideration ;  there  is,  for  instance,  the  cost  of  attendance. 
A  small  gas  engine  running  on  town  gas  will  run  without 
attention,  whereas  a  steam  boiler  requires  a  man  to  stoke  it  and 
to  see  that  the  water  level  is  maintained.  If  we  take  the  case 
of  a  10  horse-power  gas  engine  running  fifty-four  hours  per  week 
on  town  gas  costing  -6  of  a  penny  per  B.H.P.  without  attendance, 
and  a  steam  engine  costing  only  '3  of  a  penny  per  B.H.P.,  but 
necessitating  18s.  per  week  being  spent  in  wages,  the  comparison 
will  come  out  in  favour  of  the  gas  engine.  It  is  largely  due  to 
the  fact  that  small  gas  engines  can  be  run  practically  without 
attention,  and  can  be  stopped  and  started  without  any  fuel  being 
used  while  the  engine  is  standing,  that  they  are  so  widely  used 
in  small  works.  With  large  gas  engines  and  producers  the  cost 
of  attendance  is  as  great  as,  if  not  greater  than,  that  required 
with  steam  plant. 

On  grounds  other  than  those  of  running  costs  and  possible 
risk  of  boiler  explosions,  the  advantages  are  all  on  the  side  of 
the  steam  engine.  Ease  of  starting,  silence,  general  sweetness 
of  running,  and  reliability,  are  qualities  much  more  marked  in 
the  steam  engine  than  in  its  rival.  The  author  a  few  years  ago 
spent  some  hours  in  an  electric  generating  station  where  gas 
engines  were  employed,  and  he  subsequently  revisited  the  same 
station  when  the  gas  engines  had  been  replaced  by  steam  plant. 
The  strongest  impressions  produced  at  the  first  visit  were  the 
terrible  noise  (it  was  quite  impossible  to  hear  oneself  speak  in 
the  engine-room),  and  the  pallid  appearance  and  anxious  look 
upon  the  faces  of  the  attendants.  The  chief  engineer  said  that 
he  was  much  handicapped  by  illness  amongst  his  staff  due  to 
noxious  fumes. 

On  the  second  visit,  when  the  generators  were  driven  direct 
by  600  H.P.  single-acting  engines,  the  whole  conditions  were 
changed ;  one  could  converse,  without  raising  one's  voice,  in  any 
part  of  the  engine-room,  and  the  attendants  all  looked  well ; 
and  while  during  the  first  visit  one's  chief  desire  was  to 
escape  as  quickly  as  politeness  permitted  from  what  was 
almost  an  inferno,  on  the  second  visit  one  was  tempted  to 


252  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

linger  unduly  amongst  the  silent-running  plant,  and  around  the 
switch  board. 

We  have  said  that  the  steam  engine  is  more  reliable  than  the 
gas  engine.  Perhaps  this  statement  should  be  qualified  in  this 
way.  It  is  not  suggested  that  a  gas  engine,  provided  its  valves 
are  kept  clean,  and  supplied  with  a  proper  explosive  mixture,  is 
not  reliable,  but  the  author's  meaning  is  this — given  water,  coal, 
and  a  sound  boiler  one  can  be  sure  of  getting  steam  of  some 
definite  and  easily  ascertainable  pressure,  and  if  this  steam  fails 
to  drive  the  engine  the  fault  can  be  located  easily.  But  in  the 
case  of  a  producer  plant,  given  coal,  water,  and  a  producer, 
one  cannot  be  absolutely  sure  of  getting  a  gas  of  some  definite 
explosive  strength.  On  paper  it  is,  of  course,  a  very  easy 
matter,  but  in  practice  the  strength  of  the  explosive  mixture 
varies  considerably,  and  unless  the  engine  is  of  sufficient  size  to 
give  the  full  power  required  with  a  weak  mixture,  its  speed  will 
fall  off  and  the  whole  work  of  the  factory  may  be  deranged. 
A  small  gas  engine  working  with  town  gas  does  not  experience 
great  variations  in  the  quality  of  the  gas  supplied,  and  is  pro- 
bably as  reliable  as  a  steam  engine. 

Oil  Engines  work  on  the  same  principles  as  gas  engines ;  but 
the  former,  with  the  exception  of  the  Diesel  engine,  require  some 
form  of  vaporiser  to  turn  the  heavy  oils  used  into  vapour.  The 
difference  between  the  various  makes  of  oil  engines  consists 
chiefly  in  the  vaporiser.  In  the  Hornsby  engine,  as  shown  by 
the  illustration,  this  forms  a  continuation  of  the  cylinder,  and 
when  compression  is  completed  the  temperature  is  sufficiently 
high  to  fire  the  mixture  without  any  ignition  apparatus.  A 
lamp  giving  out  great  heat  is  used  at  starting,  and  is  retained 
until  the  vaporiser  has  become  sufficiently  warmed  to  vaporise 
the  oil. 

In  the  Priestman  engine  the  vaporiser  is  separate  from  the 
cylinder,  and  oil  is  vaporised  by  being  passed  through  a  fine  jet 
and  heated  by  the  exhaust  gases,  while  the  mixture  is  fired  by 
electric  ignition 

The  characteristics  of  the  Diesel  engine  have  been  referred  to 
in  the  remarks  on  ignition  and  compression. 

Petrol  Engines. — Petrol  engines  work  on  precisely  the  same 
principles  as  gas  engines.  The  four-stroke  cycle  is  generally 
adopted  in  this  country  and  abroad.  In  the  United  States,  how- 
ever, a  certain  number  of  cheap  two-stroke  cycle  engines  are 
made  for  launches.  Petrol,  owing  to  its  light  character, 
vaporises  very  easily ;  the  vaporisation  is  effected  in  a  chamber 
called  a  carburetter.  The  petrol  is  drawn  up  in  the  form  of 
spray  through  several  fine  openings  by  the  suction  of  the  engine 


GAS   AND   OIL    ENGINES. 


253 


254  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

and  mixes  with  a  current  of  air  also  drawn  up  by  the  engine 
suction. 

Consumption  of  Oil  and  Petrol. — The  amount  of  oil  used 
in  a  good  oil  engine  is  about  '085  of  a  gallon  or  *7  of  a  Ib.  per 
B.H.P.  In  the  Diesel  engine  the  consumption  is  as  low  as 
•41  Ib.  in  an  engine  of  100  H.P.,  but  the  first  cost  of  this  engine 
is  rather  high.  The  cost  of  suitable  Russian  oil  "naked  on 
wharf  "  is  about  45s.  a  ton.  The  cost  of  barrels  and  carriage  may 
bring  the  cost  up  to  «£4  per  ton,  or,  roughly,  fourpence  per  gallon. 
The  consumption  of  petrol  is  about  '1  to  '125  of  a  gallon,  or 
•7  to  -9  of  a  Ib.  per  B.H.P.  per  hour. 

Specific  Gravity  of  Oils. — We  have  spoken  of  heavy  and 
light  oils ;  the  specific  gravity  of  crude  petroleum  is  about  0-92 
so  that  a  gallon  weighs  about  9-2  Ibs.  The  ordinary  household 
petroleum  used  for  lamps  weighs  about  8- 2  Ibs.  per  gallon 
(specific  gravity  0'S2) ;  while  petrol  or  petroleum  spirit  weighs 
from  6-6  to  7 -2  Ibs.  per  gallon  (specific  gravity  0-66  to  0-72). 

One  Ib.  of  crude  petroleum  contains  about  20,000  B.T.U., 
while  1  Ib.  of  petrol  contains  about  17,500  B.T.TJ. 

Plash  Point. — The  point  at  which  an  oil  will  vaporise  when 
heated  is  termed  its  flash  point — i.e.,  if  an  oil  vaporises  at  75° 
F.  its  flash  point  is  said  to  be  75°.  An  oil,  the  flash  point  of 
which  is  below  73°,  is  not  considered  legally  safe  in  this  country, 
and  may  not  be  stored  without  restrictions;  efforts  are  being 
made  to  raise  this  flash  point  on  account  of  the  numerous  accidents 
which  occur  with  cheap  lighting  oils.  The  flash  point  of  heavy 
petroleum,  such  as  is  used  in  oil  engines,  is  about  95°. 


255 


CHAPTER  XIY. 
STRENGTH  OF  BEAMS  AND  USEFUL  INFORMATION. 

Strength  of  Beams. — To  describe  elaborate  methods  for 
calculating  with  extreme  accuracy  the  strength  of  beams  of 
unusual  section,  either  graphically  or  mathematically,  would  be 
beyond  the  scope  of  this  book,  and,  in  point  of  fact,  such 
calculations  are  seldom  made  by  the  average  draughtsman  or 
designer,  unless  engaged  upon  bridge  or  crane  work.  It  is, 
however,  necessary  for  every  young  engineer  to  be  able  to- 
ascertain  quickly  and  with  fair  accuracy,  what  stress  is  set 
up  in  the  flanges,  or  body  of  a  beam  or  cantilever  when 
loaded  in  a  given  manner. 

In  those  cases  where  the  metal  is  distributed  in  the  most 
efficient  manner  to  resist  bending — i.e.,  in  the  form  of  an  upper 
and  lower  flange  with  a  web  joining  the  two  flanges,  as  in 
the  case  of  a  rolled  steel  joist,  the  stress  in  each  flange  can  be 
found  very  quickly  and  simply  by  the  formulae  given  in 
column  1  of  Table  xxi.,  and,  if  the  area  of  the  flange  is  known, 
the  stress  per  square  inch  is  quickly  ascertained.  When  the  beam 
is  supported  at  both  ends  and  loaded,  the  upper  flange  is  in 
compression,  the  lower  flange  in  tension;  and  vice  versd  when 
the  beam  is  supported  at  one  end  only.  The  web  is  assumed 
to  give  stiffness  only  when  this  method  of  calculation  is  adopted. 

In  cases  where  the  beam  is  of  solid  rectangular  or  of  round 
section,  the  stress  cannot  be  found  so  simply,  as  the  relative 
value  of  the  metal  must  be  considered;  for  it  is  evident  that 
metal  situated  midway  between  the  top  and  bottom  of  the  beam, 
or  at  its  neutral  axis,  is  not  so  efficient  as  metal  placed  at  the 
greatest  distance  from  the  neutral  axis.  The  breaking  strength 
of  rectangular  beams  can,  however,  be  found  in  the  manner 
indicated  in  the  second  column,  and  of  beams  of  other  sections 
by  substituting  for  B  x  D2  the  values  for  these  sections  given 
by  the  figures  accompanying  the  illustrations. 

It  will  be  noticed  in  the  case  of  solid  rectangular  beams, 
column  2,  that  the  square  of  the  depth  is  taken;  the  reason  for 
this  being  that  the  strength  of  a  rectangular  beam  varies  as  the 
square  of  its  depth.  It  is  obvious,  however,  that  if  the  depth 
is  squared  and  multiplied  by  the  breadth,  the  number  of  inches 
so  found,  multiplied  by  the  safe  or  breaking  stress  of  the  metal, 


256  MECHANICAL   ENGINEERING   FOR    BEGINNERS. 

TABLE   XXI. 


Stress  in  Each  Flange 
in  Beams  of  I  Section 
(depth  measured  from 
centre  to  centre  of 
flange). 

2. 

Breaking  Weight,  in 
Cwts.,  of  Plain  Rec- 
tangular Beams. 

Greatest 
Shearing 
Force. 

A 

WL 

BD2K 

W 

ifli                    \ 

i            o 

D 

L 

Iboooooo 

WL 

2BD'K 

W 

2D 

L 

i 

WL 

4BD2K 

W 

4~  6    r 

~    4D 

L 

2 

poooooo 

_  WL 

8BD2K 

W 

te 

8D 
WL 

8BD2K 

2 
W 

^0                                 r 

8D 

T  

f) 

ip     o     F 

Jooooooi| 
P            r 

WL 

12BD2K 

W 

12D 

L 

2 

WM? 

BD2L 

W 

i       1 

LD 

LiL2 

2 

1       o^ 

W  =  weight  applie 

d. 

4-7  x  R3 

L  =  length  of  beam  in  ins. 
B  =  breadth  of  beam  in  ins. 

D  =  depth  of  beam  in  ins. 

K  =  coefficient. 

4-7  x  /rR'-rA 

=  65  to  90  cwts.  for  cast- 
iron  test  pieces  free 
from  blowholes. 

=  45    to    50    cwts.    for 

T 

BD3  -  2bd? 

ordinary    cast  -  iron 
structures. 

Q 

D 

=  90  to  95  cwts.  for  rolled 

1 

steel  joists. 

STRENGTH    OF   BEAMS   AND    USEFUL    INFORMATION.  257 

must  not  be  taken  as  giving  the  load  the  beam  will  bear, 
coefficients  for  the  ultimate  strength  of  various  materials  are 
therefore  used  when  this  method  of  calculation  is  employed,  and 
are  given  at  the  foot  of  the  table. 

The  coefficient  usually  given  for  the  strength  of  cast  iron, 
although  fairly  applicable  to  ordinary  cast  structures,  is  too  low, 
provided  the  metal  is  sound,  free  from  blowholes,  and  free  from 
internal  stresses  set  up  in  cooling.  For  instance,  we  know  that 
it  is  customary  to  specify  that  a  bar  of  cast  iron,  2  inches  deep 
1  inch  wide,  placed  on  supports  36  inches  apart,  shall  carry  a  load 
of  30  cwts.  suspended  from  its  centre,  and  occasionally  it  is 
specified  that  such  a  bar  of  cast  iron  intended  for  cylinder 
liners  shall  carry  a  load  of  40  cwts.,  and  these  results  are 
obtained  in  actual  practice.  Now,  if  we  work  by  the  formula 

— ,  which  applies  to  the  case — see  illustration  No.  3 

— and  take  a  coefficient  of  46  cwts.,  as  given  in  a  well-known 
pocket-book,  the  bar  would  break  under  a  load  of  about  20  J  cwts. 
only.  Working  by  the  rule  given  in  another  widely-read  and 
useful  pocket-book  the  bar  would  break  under  a  load  of  24  cwts. 

Some  engineers,  instead  of  taking  BD2  and  using  a  small 
coefficient,  prefer  to  take  the  useful  modulus  of  the  section, 
which  is  considered  to  be  -J-  BD2,  and  to  use  a  coefficient  six 
times  greater  than  when  using  BD2.  In  the  case  of  the  test 
bar  referred  to,  if  the  modulus  of  the  section — viz.,  £  BD2 — is 
taken,  the  coefficient  of  rupture,  assuming  the  bar  breaks  with 
30  cwts.,  is  just  over  20  tons.  If  the  bar  breaks  with  40  cwts., 
the  coefficient  of  rupture  is  about  27  tons.  These  coefficients  are 
much  higher  than  the  ultimate  tensile  stress  of  cast  iron,  and 
the  explanation  is  to  be  sought  in  the  plasticity  of  the  metal. 
Sir  Benjamin  Baker  found  that  if  a  beam  loaded  in  the  centre 
was  turned  over  and  over,  it  would  break  after  a  very  few  turns 
under  a  load,  which,  under  ordinary  circumstances,  the  beam 
would  carry  indefinitely. 

The  arrangement  of  balls  in  the  illustrations  shows  how  the 
weight  is  placed — i.e.,  whether  evenly  distributed  or  placed  in 
the  centre  of  the  beam.  In  Nos.  5  and  6  the  ends  of  the  beam 
are  built  into  the  walls  or  otherwise  fixed ;  this  'renders  the 
beam  stronger  than  if  merely  supported,  as  in  the  case  of 
Nos.  4  and  5. 

The  following  examples  will  doubtless  help  to  make  the 
formulae  clear: — 

Example  1. — The  cast-steel  arm  of  the  large  hydraulic  riveter  shown 
by  Fig.  90  is  12  feet  long  (12  feet  gap)  and  is  45  inches  deep  from  centre  to 


258 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


centre  of  flanges.     A  pressure  of  100  tons  is  applied  at  the  end  ;   what 
stress  will  there  be  in  the  flanges  ?    The  formula  -^=—  given  in  column  1 , 


100  x  144 
45 


320. 


opposite  the  first  illustration,  applies,  and  the  calculation  is 

The  stress  in  each  flange  is,  therefore,  320  tons.  The  flange  is  24  inches 
wide  and  6  inches  thick,  so  that  there  are  144  square  inches  to  withstand 
the  stress ;  the  stress  upon  the  metal  will,  therefore,  be  2*22  tons  per 
square  inch. 

Example  2. — A  rolled  steel  joist  of  I  section  6  inches  wide  and  10  inches 
deep  (say  9£  inches  from  centre  to  centre  of  flange)  is  built  securely 
into  walls  10  feet  apart,  and  is  uniformly  loaded  with  a  total  weight  of 
26'2  tons  ;  what  stress  will  there  be  in  each  flange  ?  The  formula  given 
in  column  1 ,  opposite  the  sixth  illustration,  applies,  and  the  calculation  is 

26 '2  x  120 

~m n~H~  =  27*6  tons  in  each  flange.     We  see,  from  a  sectional  drawing 

i  L  x  y  *  o 

of  the  joist  supplied  by  the  makers,  that  there  are  about  4  square  inches 
in  each  flange,  so  that  the  stress  in  the  flange  is  6 '9  tons  per  square  inch. 

Example,  3. — What  weight  suspended  from  the  centre  of  a  cast-iron  bar 
2  inches  deep  1  inch  wide,  placed  upon  supports  36  inches  apart,  would 
break  it?  The  formula  given  in  column  2,  opposite  the  third  illustration, 
applies,  and,  taking  the  coefficient  value  of  K  to  be  70  cwts.,  the  calcu- 

.„  ,     4  x  1  x  4  x  70 

lation  will  be —  -  =  31 '1  cwts. 

36 

Example  4- — What  weight  suspended  from  the  centre  of  a  cast-iron 
round  bar  3  inches  diameter,  placed  upon  supports  36  inches  apart,  would 
break  it  ?  The  value  of  the  metal  in  a  round  section  is  given  by  the  eighth 
illustration  and  accompanying  figures — viz.,  4'7  x  R3.  The  calculation, 

,,       ,         .    4  x  47  x  1-53  x  70       100 
therefore,  is — =  123  cwts. 

oO 

The  following  table,  issued  by  Messrs.  Measures  Bros.,  gives 
the  safe  permanent  loads  which  their  standard  rolled-steel  joists 
are  capable  of  bearing  when  the  ends  are  built  into  walls  10  feet 
apart,  and  when  the  load  is  evenly  distributed  : — 


Size. 

Weight  per  Foot. 

Safe  Load,  in  Tons, 
for  a  10-feet  Span. 

Inches. 

Lbs. 

6x5 

24i 

9-8 

8x5 

27 

13-0 

8x6 

30 

15-0 

9|  x  4J 

24 

140 

10    x  4£ 

m 

17-0 

10    x  5 

30 

18  6 

10    x  6 

42 

26-2 

12    x  5 

37 

27-5 

12    x  6 

45 

33-2 

14    x  6 

48 

44-1 

16    x  6 

62 

63-0 

STRENGTH    OF    BEAMS    AND    USEFUL    INFORMATION.  259 

As  the  strength  of  a  beam  varies  inversely  as  its  length,  it  is 
easy  to  calculate  what  a  beam  of  a  length  other  than  10  feet  will 
carry;  thus,  a  beam  20  feet  between  walls  will  carry  half  the 
weight  given  above.  If  the  ends  of  the  beam  are  merely  sup- 
ported, and  not  built  into  the  walls,  the  beam  will  only  carry 
two-thirds  of  the  weight  given  above  (or  in  the  ratio  of  8  to  12). 
The  ratio  of  strengths  of  beams,  variously  loaded  and  supported, 
will  be  seen  by  glancing  at  the  illustrations  and  the  formulae  in 
columns  1  and  2. 

In  connection  with  structural  ironwork,  the  student  may 
occasionally  find  it  useful  to  be  able  to  ascertain  the  stress  in 
any  part  of  a  structure  by  the  graphic  method.  Fig.  104  shows 
this  method  applied  to  a  jib  crane  having  a  weight  suspended 
from  a  hook  at  the  end.  Assuming  the  weight  suspended  from 
A  is  3  tons,  and  it  is  desired  to  know  what  stress  will  be  set  up 
in  the  jib  and  tie  bars,  the  method  of  procedure  is  as  follows  : — 
Draw  A  B  equal  to  the  weight ;  thus,  if  the  weight  is  3  tons,  the 
line  on  a  large-scale  drawing  may  be  drawn  3  inches  long.  Draw 
the  horizontal  line  B  C,  then  C  A  will  show  what  stress  there 
is  in  the  jib — i.e.,  if  C  A  is  4  inches  long,  the  stress  in  the  jib  is 
4  tons.  To  find  the  stress  in  the  tie  bars,  draw  the  vertical  line 
C  D,  then  D  A  will  show  the  stress  in  the  tie  bars.  If  the  bars 
had  been  horizontal,  as  indicated  by  dotted  lines,  the  stress  in 
them  would  have  been  shown  by  E  A.  The  stress  in  the  jib  and 
tie  bars  may  also  be  found  by  simple  arithmetic  ;  thus,  the  stress 
in  the  jib  is  found  by  multiplying  the  weight  carried  by  the 
length  of  the  jib,  and  dividing  by  the  distance  F  G  (not  F  X). 
The  stress  in  the  bars  is  found  by  multiplying  the  weight  carried 
by  the  length  of  the  bars,  and  dividing  by  the  distance  F  G. 
The  student  would  do  well  to  draw  out  a  triangle  to  a  fairly 
large  scale,  take  an  imaginary  weight,  and  check  the  graphical 
method  by  arithmetical  calculations. 

The  extremely  simple  methods  given  above  apply  only  if  the 
weight  is  suspended  from  the  end  of  the  jib.  If  the  chain 
carrying  the  weight  passes  along  the  tie  bars  and  is  pulled  at  by 
a  winch,  then  it  is  necessary  to  take  the  resultant  force  which 
arises  from  the  pull  on  the  chain  at  one  end  and  the  weight  at 
the  other.  This  resultant  force  is  found  very  easily  by  the 
graphical  method,  as  shown  by  Fig.  105.  The  chain  is  shown  by 
the  dotted  line ;  mark  off  A  B  equal  to  the  weight,  and  mark  off 
A  C  equal  to  the  pull  upon  the  chain ;  as  the  stress  in  all  parts 
of  the  chain  is  the  same,  it  is  obvious  that  AC  =  AB.  Complete 
the  parallelogram  C  A  B  D,  and  the  resultant  force  is  shown  by 
the  line  D  A.  If  now  we  wish  to  find  the  stresses  set  up  in  the 
tie  bars  and  jib  of  a  crane  in  which  the  chain  passes  along  the 


260 


MECHANICAL    ENGINEERING    FOR    BEGINNERS. 


tie  bars  the  method  of  doing  so  is  shown  by  Fig.  106.  The 
stress  in  the  tie  bars  is  shown  by  the  line  M,  and  in  the  jib  by 
the  line  P. 

£•       26 


Fig  105.  Fig.  106. 

Figs.  104  to  106. — Graphic  methods  of  computing  stresses. 

A  point  to  be  borne  in  mind  in  the  construction  of  cranes  is 
this — If  the  angle  formed  by  the  more  or  less  horizontal  portion 
of  the  chain  and  the  jib  is  greater  than  the  angle  formed  by  the 


STRENGTH  OF  BEAMS  AND  USEFUL  INFORMATION.     261 

jib  and  vertical  portion  of  the  chain,  then  the  tie  bars  will  be  in 
compression  ;  if  less,  then  the  tie  bars  will  be  in  tension. 

Amongst  the  wharfs  and  warehouses  of  big  cities  one  occasion- 
ally sees  cranes  with  the  tie  bars  bent.  This  is  because  such 
cranes  were  designed  by  men  without  the  knowledge  of  the 
simple  fact  just  mentioned;  and  small  round  tie  bars,  which 
would  be  quite  suitable  for  a  tensile  stress,  have  been  subjected 
to  compression  and  have  consequently  bent.  The  author  met 
one  crane-maker  who  refused  to  believe  that  the  tie  bars  of  a  jib 
crane  could  ever  be  in  compression,  and  it  was  not  until  ocular 
proof  was  given  by  means  of  a  lawn-tennis  post  placed  as  a  jib, 
and  a  piece  of  cord  and  weight  representing  the  chain,  that  the 
crane-maker  was  convinced.  He  had  attributed  the  fact  of  the 
tie  bars  of  his  cranes  becoming  bent  to  the  practice  of  men 
crawling  along  them  to  oil  the  sheave  at  the  end  !  If  the  jib  is 
placed  at  an  angle  of  45°  both  in  respect  to  the  horizontal  and 
vertical  portions  of  the  chain,  then  the  resultant  thrust  is  entirely 
taken  by  the  jib,  and  the  tie  bars  are  neither  in  tension  nor  com- 
pression ;  but,  of  course,  any  swing  of  the  chain  will  upset  this 
equilibrium  at  once. 

The  pull  on  the  wall  at  the  top  bracket  of  a  crane  equals  the 
weight  multiplied  by  the  radius  of  the  crane,  and  divided  by  the 
height  of  the  crane  post. 

Sundry  Useful  Information.  —  All  questions  relating  to  the 
power  transmitted  by  a  screw,  lever,  wedge,  or  toggle  joint,  can 
be  answered  by  the  following  formula  :  — 


-r» 


p  m  P  M 


where  P  =  power  transmitted. 
p  =  power  applied. 
M  =  motion  of  power  transmitted. 
m  —         ,,  „        applied. 

Example.  —  The  hand-  wheel  of  a  stop  valve  is  30  inches  in  circumference, 
and,  for  the  sake  of  a  simple  example,  we  will  assume  that  it  is  keyed  to  a 
screwed  spindle  having  only  one  thread  per  inch,  so  that  during  one 
revolution  of  the  hand-  wheel  the  valve  will  be  raised  or  lowered  1  inch. 
A  force  of  10  Ibs.  is  exerted  at  one  portion  of  the  hand-wheel,  what  power 
will  be  transmitted  to  the  valve  ?  The  power  applied  will  move  through 
30  inches,  while  the  power  transmitted  will  move  through  1  inch.  The 

calculation,  therefore,  is  —  —  =300.     The  power  transmitted  to  the 

valve  is  300  Ibs. 

Questions  arising  in  connection  with  the  use  of  levers  can  be 


262  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

answered  by  the  help  of  the  following  formula  (the  formula  is 
transposed  for  the  convenience  of  beginners) : — 


W 


y 

where  W  =  weight. 

=  its  leverage. 

pressure. 
x  =  its  leverage. 


y 
P 


Example  1.  —  A  weight  of  20  Ibs.  is  placed  at  the  end  of  a  safety-valve 
lever  12  inches  from  the  fulcrum  ;  the  lever  acts  on  the  valve  at  a  distance 
of  1£  inches  from  the  fulcrum  ;  what  pressure  will  be  exerted  on  the  valve  ? 
The  formula  is 


, 

Example  2.  —Suppose  the  pressure  thus  obtained  is  not  sufficiently  great, 
and  we  require  a  pressure  of  250  Ibs.  on  the  valve,  how  long  must  the  lever 
be,  keeping  the  weight  the  same  ?  The  formula  transposed  to  meet  this 
case  is 

P  x  a:  250  x  1-5 

-W-=y'°r—  20~   =18'75' 

The  lever  must,  therefore,  be  18  '75  inches  long. 

Thermometer  Scales.  —  The  reader  will  find  in  the  technical 
press  and  elsewhere  that  temperatures  are  frequently  given  in 
degrees  Centigrade.  To  convert  degrees  Centigrade  into  degrees 
Fahrenheit  it  is  necessary  to  multiply  by  9,  divide  by  5,  and  add 
32  to  the  result.  Thus  — 

100°  C.  212°  F. 

9  -    32 
f.  \  Qnn              To  convert  degrees  Fahren- 

0  )  *™              heit  into  degrees  Centigrade,  Ly~ 

proceed  thus— 

+    32  9  )  900 


212°  F.  100°. 

The  reader  will  see  from  the  above  examples  that  100°  C.  are 
equivalent  to  212°  F. 

Boiler  Pressures. — In  France  pressures  are  spoken  off  in 
terms  of  kilogrammes  per  square  centimetre.  One  kilo,  per 
square  centimetre  =  14'2  Ibs.  per  square  inch,  or  a  little  less 


STRENGTH    OF    BEAMS   AND    USEFUL    INFORMATION. 


263 


than  one  atmosphere.  It  is  useful  to  remember  this,  as  it 
enables  one  to  make  a  rough  mental  calculation  as  to  the  pressure 
in  pounds  per  square  inch,  when  it  is  referred  to  in  the  other 
and  less  familiar  way. 

To  Divide  a  Straight  Line  into  a  Number  of  Equal 
Parts. — It  is  frequently  necessary  to  divide  a  line  of  some 
uneven  length  into  a  number  of  equal  parts.  For  instance,  let 
us  suppose  that  the  line  A  B  =  2±%  inches  long,  and  we  wish  to 


8 


divide  it  into  10  equal  parts.     The  best  way  to  proceed  is  as 
follows: — 

Draw  A  C  at  any  convenient  angle  and  of  any  length.  Divide 
this  line  into  10  parts  of  equal  length;  it  does  not  matter  what 
the  length  is  so  long  as  the  divisions  are  all  equal.  Draw  a  line 
from  the  10th  point  to  B,  and  lines  from  the  other  points  of 
division  parallel  to  10  B.  A  B  will  then  be  divided  by  these 
lines  into  ten  equal  parts. 


264 


MECHANICAL    ENGINEERING   FOR    BEGINNERS. 


TABLE  XXII. — DECIMAL  EQUIVALENTS  OF  AN  INCH  WITH 
AREAS  AND  CIRCUMFERENCES. 


Fraction 
of  an 
Inch. 

Decimals 
of  an  Inch. 

Area. 

Circumference. 

TV 

•0625 

•00307 

•1963 

i 

•125 

•01227 

•3927 

A 

•1875 

•02761 

•589 

i 

•25 

•04909 

•7854 

A 

•3125 

•0767 

•9817 

i 

•375 

•1104 

1-1781 

T*B 

•4375 

•1503 

1-3744 

1 

•5 

•1963 

1-5708 

& 

•5625 

•2485 

1-771 

| 

•625 

•3068 

1-9635 

tt 

•6875 

•3712 

2-1598 

I 

•75 

•4417 

2-3562 

H 

•8125 

•5185 

2-5525 

I 

•875 

•6013 

2-7489 

•  tt 

•9375 

•6903 

2-9452 

i 

1-0 

•7854 

3-1416 

Inches. 

Decimals 
of  a  Foot. 

Inches. 

Decimals 
of  a  Foot. 

1 

•0833 

7 

•5833 

2 

•1667 

8 

•6667 

3 

•25 

9 

•75 

4 

•3333 

10 

•8333 

5 

•4167 

11 

•9167 

6 

•5 

12 

1-0 

STRENGTH    OF    BEAMS    AND    USEFUL    INFORMATION.  265 


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268 


MECHANICAL    ENGINEEKING    FOR    BEGINNERS. 


TABLE   XXIV.— TABLE  OF  SQUARES  AND  CUBES. 


NV 

Square. 

Cube.  II  Xo. 

Square. 

Cube. 

Mo. 

Square. 

Cube. 

So. 

Square. 

Cube.   ' 

1 

1 

1 

51 

2,601 

132,651 

101 

10,201 

1,030,301 

151 

22,801 

3,442,951 

2 

4 

8 

52 

2,704 

140,608 

102 

10,404 

1,061,208 

152 

23,104 

3,511,808 

3 

9 

27 

53 

2,809 

148,877 

103 

10,609 

1,092,727 

153 

23,409 

3,581,577 

4 

16 

64 

54 

2,916 

157,464 

104 

10,816 

1,124,864 

154 

23,716 

3,652,264 

5 

25 

125 

55 

3,025 

166,375 

105 

11,025 

1,157,625 

155 

24,025 

3,723,875 

8 

36 

216 

56 

3,136 

175,616 

10(5 

11,236 

1,191,016 

156 

24,336 

3,796,416 

7 

49 

343 

57 

3,249 

185,193 

107 

11,449 

1,225,043 

157 

24,649 

3,869,893 

8 

64 

512 

58 

3,364 

195,112 

108 

11,664 

1,259,712 

158 

24,964 

3,944,312 

9 

81 

729 

59 

3,481 

205,379 

109 

11,881 

1,295,029 

159 

25,281 

4,019,679 

10 

100 

1,000 

60 

3,600 

216,000 

110 

12,100 

1,331,000 

160 

25,600 

4,096,000 

11 

121 

1,331 

61 

3,721 

226,981 

111 

12,321 

1,367,631 

165 

27,220 

4,492,120 

12 

144 

1,728 

62 

3,844 

238,328 

112 

12,544 

1,404,928 

170 

28,900 

4,913,000 

13 

169 

2,197 

63 

3,969 

250,047 

113 

12,769 

1,442,897 

175 

30,620 

5,359,370 

14 

196 

2,744 

64 

4,096 

262,144 

114 

12,996 

1,481,544" 

180 

32,400 

5,832,000 

15 

225 

3,375 

65 

4,225 

274,625 

115 

13,225 

1,520,875" 

185 

34,220 

6,331,600 

16 

256 

4,096 

66 

4,356 

287,496 

116 

13,456 

1,560,896 

190 

36,100 

6,859,000 

17 

289 

4,913 

67 

4,489 

300,763 

117 

13,689 

1,601,613 

195 

38,020 

7,414,900 

18 

324 

5,832 

68 

4,624 

314,432 

118 

13,924 

1,643,032 

200 

40,000 

8,000,000 

19 

361 

6,859 

69 

4,761 

328,509 

119 

14,161 

1,685,159 

210 

44,100 

9,261,  00 

20 

400 

8,000 

70 

4,900 

343,000 

120 

14,400 

1,728,000 

220 

48,400 

10,684,000 

21 

441 

9,261 

71 

5,041 

357,911 

121 

14,641 

1,771,561 

230 

52,900 

12,167,000 

22 

484 

10,648 

72 

5,184 

373,248 

122 

14,884 

1,815,848 

240 

57,600 

13,824,000 

28 

529 

12,167 

73 

5,329 

389,017 

123 

15,129 

1,860,867 

250 

62,500 

15,625,000 

24 

576 

13,824 

74 

5,476 

405,224 

124 

15,376 

1,906,624 

260 

67,900 

17,576,000 

25 

625 

15,625 

75 

5625 

421,875 

125 

15,625 

1,953,125 

270 

72,900 

19,683,000 

26 

676 

17,576 

76 

5,776 

438,976 

126 

15,876 

2,000,376 

280 

74,840 

21,952,000 

27 

729 

19,683 

77 

5,929 

456,533 

127 

16,129 

2,048,383 

290 

84,100 

24,389,000 

28 

784 

21,952 

78 

6,084 

474,552 

128 

16,384 

2,097,152 

300 

90,000 

27,000,000 

•29 

841 

24,389 

79 

6,241 

493,039 

129 

16,641 

2,146,689 

310 

96,100 

29,791,000 

30 

900 

27,000 

80 

6,400 

512,000 

130 

16,900 

2,197,000 

320 

102,400 

32,768,000 

31 

961 

29,791 

81 

6,561 

531,441 

131 

17,161 

2,248,091 

330 

108,900 

35,937,000 

32 

1,024 

32,768 

82 

6,724 

551,368 

132 

17,424 

2,299,968 

340 

115,600 

39,340,000 

33 

1,089 

35,937 

83 

6,889 

571,787 

133 

17,689 

2,352,637 

350 

122,500 

42,575,000 

34 

1,156 

39,304 

84 

7,056 

592,704 

134 

17,956 

2,406,104 

360 

129,600 

46,656,000 

35 

1,225 

42,875 

85 

7,225 

614,125 

135 

18,225 

2,460,375 

370 

136,900 

50,653,000 

36 

1,296 

46,656 

86 

7,396 

636,056 

136 

18,496 

2,515,456 

380 

144,400 

54,872,000 

37 

1,369 

50,653 

87 

7,569 

658,503 

137 

18,769 

2,571,353 

390 

152,100 

59,319,000 

38 

1,444 

54,872 

88 

7,744 

681,472 

138 

19,044 

2,628,072 

400 

160,000 

64,000.000 

39 

1,521 

59,319 

89 

7,921 

704,969 

139 

19,321 

2,685,619 

410 

168,100 

68,921,000 

40 

1,600 

64,000 

90 

8,100 

729,000 

140 

19,600 

2,744,000 

420 

176,400 

74,088,000 

41 

1,681 

68,921 

91 

8281 

753,571 

141 

19,881 

2,803,221 

1430 

184,900 

79,507,000 

42 

1,764 

74,088 

92 

8,464 

778,688 

142 

20,164 

2,863,288 

440 

193,600 

85,184,000 

4o 

1,849 

79,507 

93 

8,649 

804,357 

143 

20,449 

2,924,210 

450 

202,500 

91,125,000 

44 

1,936 

85,184 

94 

8,836 

830,584 

144 

20,736 

2,985,980 

460 

211,600 

97,336,000 

45 

2,025 

91,125 

95 

9,025 

857,375 

145 

21,025 

3,048,620 

470 

220,900 

103,823,000 

46 

2,116 

97,336 

96 

9,216 

884,736 

146 

21,316 

3,112,136 

480 

230,400 

110,572,000 

47 

2,209 

103,823 

97 

9,409 

912,673 

147 

21,609 

3,176,523 

490 

240,100 

117649,000 

48 

2,304 

110,592 

98 

9,604 

941,192 

148 

21,904 

3,241,792 

500 

250,000 

125,000,000 

49 

2,401 

117,649 

99 

9,801 

970,299 

149 

22,201 

3,307,949 

600 

360,000 

216,000,000 

50 

2,500 

125,000 

100 

10,000 

1,000,000 

150 

22.500 

3,375,000 

700 

490,000 

343,000,000 

i 

STRENGTH    OP    BEAMS    AND    USEFUL    INFORMATION.  269 

WEIGHTS  AND  MEASURES  WITH  METRIC  EQUIVALENTS. 

LENGTH. 

1  inch  =    25-399  millimetres. 
12  inches  =  1  foot   =  304*799 
3  feet      =  1  yard  =  914-391 

,,      =         '914  metre. 
1,760  yards  =  1  mile  =      1'609  kilometres. 

1  metre  =       39 '3704  inches. 

1  decimetre   =         3 '9370       ,, 
1  centimetre  =  '3937  inch. 

1  millimetre  =  '0393     ,, 

1  kilometre    =  1,093  yards. 

WEIGHT. 

1  ounce  =    28 -35  grams. 

16  ounces                   =  1  pound  .         =  453 -59      ,, 

1      ,,  =         -453  kilogram. 

28  pounds                  =  1  quarter  =    12*7      kilograms. 

4  quarters                =  1  hundredweight  (112  Ibs. )  =    50'8  ,, 

20  hundredweights  =  I  ton  (2,240  Ibs.)  =  1,016 

1  kilogram  =  2-2046  Ibs. 
1  gram         =     "0022  Ib. 

AREA. 

1  sq.  inch    =  645  sq.  millimetres. 

144  sq.  inches  =  1  sq.  foot    =         '0929  sq.  metre. 

9  sq.  feet       =  1  sq.  yard  =         '8315 
4,840  sq.  yards  =  1  acre          =         -4046  Hectare. 

1  sq.  metre  =  10  '764  feet. 

VOLUME. 

1  cubic  inch  =  16*387  cubic  centimetres 
1,728  cubic  inches  =  1     ,,      foot  =       -0283   ,,     metre. 
27     ,,     feet      =1     ,,      yard  =       '7645   ,, 

cubic  metre  =    35*3148  cubic  feet. 

=      1-3079     „     yards. 
,,     centimetre  =        "061       ,,     inch, 
gallon  =        -1605     ,,     foot. 

,,  =  277-27         ,,     inches. 

=      4-54      litres. 


271 


CONCLUSION. 

A  FEW  words  of  advice  from  one  who  has  been  through  the  mill 
may  perhaps  not  be  taken  amiss. 

In  the  first  place,  a  man  who  has  "  got  it  all  down  in  a  book  at 
home  "  is  not  of  very  much  use  in  a  drawing  office,  as  questions 
usually  require  to  be  settled  when  they  arise.  Secondly,  it  is 
quite  inadmissible  to  have  text-books  lying  around  one's  draw- 
ing board.  Engineering  pocket-books  containing  tables  of  areas, 
squares,  &c.,  are,  however,  permitted,  and  as  they  usually  contain 
some  blank  pages  at  the  end,  it  is  a  good  plan  to  copy  into  them 
any  formulae  or  rules  which  the  student  thinks  may  be  likely 
to  be  useful  to  him.  If  the  beginner  has  not  got  one  of  these 
pocket-books,  then  he  should  copy  into  a  small  note-book  of  his 
own  any  useful  information  which  he  may  come  across.  Such 
a  note-book,  to  be  of  any  real  use,  should  be  small  and  thin,  so 
that  it  may  be  carried  in  the  pocket  without  inconvenience. 

Amongst  the  information  which  a  young  draughtsman  should 
have  available  at  a  moment's  notice,  the  following  may  be  men- 
tioned : — Power  transmitted  by  belts,  ropes,  and  shafts  ;  rules  as 
to  size  of  steam  pipes ;  areas  of  chimneys ;  head  necessary  to 
overcome  friction  of  water  in  pipes ;  stress  set  up  in  the  rim  of 
a  flywheel  due  to  centrifugal  force;  stresses  in  beams,  &c.  A 
young  draughtsman  who  can  settle  questions  such  as  these,  at  all 
events  in  connection  with  preliminary  drawings  such  as  are 
frequently  sent  out,  will  have  a  much  better  chance  of  promotion 
than  one  who  is  continually  troubling  his  superior  with  questions 
such  as,  "How  wide  must  I  make  the  pulley?"  "What  size 
must  the  steam  pipe  be?''  "How  large  a  chimney  shall  I 
show?"  <fec.,  &c. 

The  next  question  which  arises  is — How  much  mathematical 
knowledge  is  it  necessary  for  a  young  engineer  to  have  in  order 
to  succeed?  The  reply  is — The  more  knowledge  one  has,  the 
better  one  is  equipped  for  one's  work ;  but  if, 'in  the  acquirement 
of  such  knowledge,  one  loses  other  qualifications  for  success, 
such  as  health,  good  eyesight,  strength  of  character,  and  robust 
commonsense,  then  the  acquirement  of  profound  mathematical 
knowledge  may  be  too  dearly  bought. 

The  brain  of  a  highly-trained  mathematical  man  may  not 
unfitly  be  compared  to  a  razor,  while  that  of  the  ordinarily 
successful  engineer  and  man  of  business  to  an  axe,  which  is  not 
easily  turned  aside  by  opposition.  If  an  individual  is  compelled 


272  MECHANICAL    ENGINEERING   FOR    BEGINNERS. 

by  force  of  circumstances  to  go  out  to  the  backwoods,  make  his 
own  clearing,  and  build  his  own  house,  there  can  be  little  doubt 
which  of  the  two  instruments  will  prove  of  most  service.  The 
author  has  no  desire  to  decry  the  razor,  but,  at  the  same  time, 
he  would  not  recommend  a  young  man  with  his  own  clearing  to 
make  to  attempt  to  grind  and  temper  his  comparatively  blunt, 
but  serviceable,  instrument  until  it  had  the  edge  of  a  razor,  but 
at  the  expense  of  other  and  more  valuable  qualities. 

The  late  Lord  Thring  stated  he  "found  that  the  apparent 
object  of  legal  expression  was  to  conceal  the  meaning  from 
ordinary  readers,"  and  although  the  author  has  great  respect  for 
the  learning  of  Professors  of  Engineering,  he  cannot  help  feeling 
that  the  methods  of  many  of  them,  while  not  designed  to 
conceal  their  meaning,  yet  are  calculated  to  deter  the  ordinary 
youth  from  studying  the  theoretical  side  of  engineering  at  all. 

Take,  for  instance,  the  following  statement  which  appears  in 
a  book  intended  for  young  students  of  engineering  : — "  In  single 

shear,  the  shearing  resistance  of  the  rivet  is  j  d2  f  ;  where  f  is 

the  resistance  of  the  material  to  shearing."  Would  it  not  have 
been  simpler  to  state  that  in  single  shear  the  shearing  resistance 
of  the  rivet  is  A  x  R ;  where  A  =  area,  R  =  resistance  of  the 

metal  to  shearing  ?  An  expression  such  as  '-  d 2  f  is  all  very 
well  for  professors  and  others  who  can  see  at  a  glance  that  —  o?2 

stands  for  area;  but  if  a  conscientious  beginner  laboriously 
squares  the  diameter  of  the  rivet,  multiplies  the  result  by  3 *141, 
then  divides  by  4,  only  to  find  that  the  whole  of  this  labour 
conveys  the  remarkable  intelligence  that  the  strength  of  a  rivet 
equals  its  area  multiplied  by  the  ability  of  the  metal  to  resist 
shear,  he  will  probably  say  that  his  own  commonsense  would 
tell  him  that,  and  will  struggle  no  further  with  other  formulae 
which  might  really  be  of  use.  If  the  beginner  thinks  any  more 
about  the  subject,  he  will  wonder  why  in  the  name  of  common- 
sense  those  gentlemen  who  are  so  fond  of  using  Greek  symbols 
at  every  possible  opportunity,  do  not  choose  one  to  represent  the 

*7T 

area  of  a  circle,  and  stick  to  it,  instead  of  writing  -r  d  -. 

Again,  if  a  student  should  ask  a  junior  professor  how  to  find 
the  pressure  on  the  slide  bars  of  an  engine,  he  will  probably  be 
told  that  the  pressure  can  be  found  by  multiplying  the  pressure 
on  the  piston  by  the  sine  of  the  angle  formed  by  a  prolongation 
of  the  piston-rod  and  connecting-rod  !  Now,  in  a  drawing  office 
where  draughtsmen's  time  has  to  be  paid  for,  one  does  not  find 


CONCLUSION.  273 

them  seeking  out  angles  by  means  of  protractors,  and  then 
turning  up  tables  of  sines,  cosines,  secants,  &c.  If  a  draughts- 
man wishes  to  know  what  the  greatest  pressure  will  be  upon  the 
slide  bars,  he  simply  multiplies  the  pressure  exerted  by  the 
piston  by  the  length  of  crank,  and  divides  by  the  length  of 
connecting-rod,  ignoring  sines  and  angles  altogether. 

As  an  example,  showing  the  effect  of  a  beginner  trying  to 
work  out  simple  problems  by  advanced  geometrical  methods,  of 
which  he  has  only  an  imperfect  grip,  a  short  narrative  of  what 
occurred  in  a  Westminster  drawing  office,  in  which  the  author 
worked  as  a  junior,  may  not  be  out  of  place.  In  the  office  in 
question,  it  was  necessary  frequently  to  run  out  the  stress  set  up 
in  riveter  arms.  So  far  as  the  author  was  privileged  to  see,  no 
calculations  more  elaborate  than  those  described  in  the  preceding 
chapter  were  ever  made,  either  by  the  chief  draughtsman  or  his 
assistants.  In  fact,  a  tradition  was  current  in  the  office  that  the 
riveter  arms  had  in  the  first  instance  been  designed  by  a  gentle- 
man (not  the  inventor)  on  ultra-scientific  lines,  and  had  all 
broken.  However,  whether  this  tradition  is  or  is  not  true,  it  is 
beside  the  story,  which  is  this : — There  came  to  the  drawing 
office,  as  a  premium  pupil,  a  young  man  who  had  had  the  benefit 
of  some  training  at  a  technical  college.  This  pupil  looked  upon 
the  ordinary  office  methods  of  arriving  at  the  stress  set  up  in  a 
riveter  arm  with  considerable  scorn.  He  said  they  were  quite 
unscientific,  and  that  he  would  show  the  correct  way.  The  next 
time  a  riveter  of  extra  large  gap,  or  extra  heavy  power,  was 
required,  the  problem  was  given  to  the  pupil,  as  well  as  to  one  of 
the  older  men,  to  work  out.  In  the  case  of  the  pupil,  the  result 
was  as  follows  : — After  some  hours  of  steady  work  a  diagram  was 
produced,  which  to  the  other  juniors  appeared  to  be  like  a  small 
church  with  its  steeple.  For  the  success  of  the  calculations, 
apparently,  it  was  necessary  to  find  with  great  accuracy  the 
centre  of  gravity  of  the  diagram.  The  little  church  was,  there- 
fore, cut  out  in  cardboard,  and  attempts  made  to  find  the  centre  of 
gravity  by  balancing  it  on  the  point  of  a  divider.  Unfortunately, 
just  as  the  desired  centre  was  nearly  found,  the  office  door  would 
be  opened,  a  current  of  air  would  cause  the  diagram  to  fall  to  the 
ground,  and  operations  had  to  be  recommenced.  However,  the 
centre  of  gravity  having  at  last  been  found,  more  steady  work 
was  put  in,  a  book  of  logarithms  was  freely  used,  several  sheets 
of  foolscap  were  covered  with  figures,  and  at  last  the  pupil  said 
his  calculation  showed  that  the  arm  would  require  to  be  (as 
nearly  as  the  author  can  remember)  about  5,283  feet  deep  ! 
When  it  was  pointed  out  that  a  riveter  arm  over  a  mile  across 
would  be  rather  unusual,  the  pupil  remarked,  quite  unabashed, 

18 


274  MECHANICAL    ENGINEERING   FOR    BEGINNERS. 

that  he  supposed  he  had  got  wrong  with  his  logarithms.  The 
strangest  feature  about  this  pupil  was  that,  although  he  obtained 
contradictory  results  every  time  he  worked  out  the  same  problem, 
yet  he  remained  convinced  that  his  was  not  only  the  right  and 
scientific  way  of  working,  but  also  the  best  way.  Possibly  it 
was  for  one  who  really  understood  it,  and  had  the  time  and 
ability  to  work  out  the  problem  correctly,  but  the  method  in 
question  was  quite  unsuitable  for  every-day  use  in  a  drawing 
office. 

If  the  student  does  not  take  kindly  to  advanced  mathematics 
he  need  not  be  discouraged ;  only  those  who  have  actually 
worked  in  drawing  offices  of  good  and  successful  firms  know 
how  small  a  part  mathematics  play  in  the  daily  life  of  the  office. 
The  problem  is  rather  how  to  get  out  the  work  quickly  with  a 
limited  staff  and  to  avoid  mistakes,  than  to  work  out  elaborate 
calculations. 

The  fundamental  difference  between  work  in  the  drawing 
office  of  a  technical  college  and  work  done  in  a  real  drawing 
office,  is  that  a  mistake  made  in  the  former  does  not  really 
matter  and  may  possibly  pass  unnoticed,  while  a  mistake  made 
in  the  latter  may  involve  a  heavy  pecuniary  loss,  or  even  give 
rise  to  a  Coroner's  inquest.  In  the  drawing  office  of  a  technical 
college  the  omission  to  mark  the  revolving  part  of  a  certain 
machine  "cast  steel,"  leaving  it  to  be  inferred  that  it  was  to 
be  of  cast  iron,  might  result  in  the  loss  of  a  few  marks.  In 
a  Midland  town  recently,  such  an  omission  on  the  part  of  a 
draughtsman  caused  the  loss  of  a  couple  of  lives.  Had  the 
draughtsman  been  accustomed  to  run  out  by  simple  arithmetic 
the  stresses  set  up  in  a  flywheel  rim,  as  described  in  a  previous 
chapter,  this  accident  and  the  resulting  inquest,  verdict,  and 
somewhat  disagreeable  rider,  might  have  been  avoided.  Un- 
fortunately many  draughtsmen  are  deterred  from  making 
calculations,  which,  although  really  simple,  are  made  to  appear 
difficult  and  complicated  by  so  many  text-books. 

If  the  young  engineer  should  eventually  have  a  business  of  his 
own,  his  problems  will  be,  not  how  to  make  elaborate  mathema- 
tical calculations,  but  how  to  get  remunerative  work,  and,  when 
obtained,  how  to  turn  it  out  quickly  and  well.  In  many  cases 
another  problem  will  be  added — viz.,  where  to  find  the  money  for 
the  following  week's  wages  for  his  men.  If  a  knowledge  of  the 
Calculus  would  enable  a  young  engineer  to  solve  this  problem 
satisfactorily  upon  all  occasions,  there  is  but  little  doubt  that 
it  would  be  studied  much  more  widely  than  is  at  present  the 
case. 

It  must  be  borne   in  mind    that,   valuable  as   mathematical 


CONCLUSION.  275 

investigations  are  in  enabling  one  to  understand  a  subject 
thoroughly,  they  usually  follow,  and  do  not  precede,  useful  in- 
ventions ;  for  instance,  had  not  such  comparatively  "ignorant" 
men  as  Watt  and  George  Stephenson  invented  and  perfected  the 
steam  engine,  many  of  our  modern  Professors,  who  write  so 
learnedly  and  brilliantly  on  the  subject,  would  have  been  com- 
pelled to  devote  their  abilities  to  astronomy  or  kindred  subjects. 
Again,  let  us  take  the  case  of  the  problem  of  aerial  navigation, 
which,  at  the  moment  of  writing  these  pages,  has  not  been 
solved.  Books  showing  mathematically  and  theoretically  how 
such  flight  may  be  accomplished  seem  to  be  strangely  lacking, 
but  as  soon  as  some  possibly  ignorant  and  benighted  inventor 
constructs  an  air  ship  which  can  be  used  with  safety,  then, 
undoubtedly,  the  technical  press  will  teem  with  books  filled 
from  cover  to  cover  with  mathematics  showing  exactly  how  it 
is  done,  and  the  student  will  be  told  that,  if  he  wishes  to  know 
anything  about  aerial  flight,  he  must  master  the  contents  of 
these  works.  One  cannot  help  feeling,  however  unreasonable 
the  feeling  may  be,  that  such  books  would  be  still  more  useful  if 
they  were  published  before,  rather  than  after,  the  problem  has 
been  solved  on  other  lines. 

The  author  has  often  been  struck  with  the  fact  that  when  an 
engineering  firm  gets  into  a  slight  difficulty,  say  the  ropes  will 
persist  in  jumping  off  a  certain  pulley,  or  a  barometric  condenser 
will  not  give  a  vacuum,  or  possibly  a  boiler  isolating  valve 
knocks  itself  to  pieces,  it  is  not  the  man  with  the  greatest 
theoretical  and  mathematical  knowledge  who  gets  over  the 
trouble,  but  in  nearly  every  case  it  is  some  one  having  the 
irreducible  minimum  of  mathematical  knowledge.  The  reason 
for  the  success  of  the  one  man  and. the  failure  of  the  other  is 
probably  due  to  the  fact,  not  that  the  one  man  is  ignorant  of 
mathematics,  but  that  he  can  think  and  reason  upon  the  facts 
before  him  undisturbed  by  the  noise  of  machinery  and  uncom- 
fortable surroundings,  while  the  other  can  only  reason  and  think 
in  symbols,  and  when  seated  in  a  quiet  room  with  a  clean  sheet 
of  paper  before  him. 

In  the  author's  opinion,  there  is  one  qualification  for  success 
which  is  of  considerably  more  importance  than  a  knowledge  of 
advanced  mathematics ;  it  is,  to  put  it  colloquially,  the  ability  to 
"keep  on  keeping  on."  It  was  entirely  by  his  determination 
and  strength  of  purpose  that  the  late  Mr.  Tweddell  was  able  to 
make  the  world  see  the  value  of  his  hydraulic  riveter,  and  to 
reap  the  financial  reward  due  to  him.  The  same  may  be  said  of 
Mr.  Parsons  and  his  turbine,  and  of  many  others. 

Competition  is  getting  keener  every  day,  and  the  struggle  for 


276  MECHANICAL    ENGINEERING    FOR    BEGINNERS. 

a  competence  more  severe,  but  there  is  a  good  deal  of  truth  in 
the  brave  words  of  a  modern  writer — "  Between  aspiration  and 
achievement  there  is  no  great  gulf  fixed,  only  the  faith  of  con- 
centrated endeavour,  only  the  stern  years  which  must  hold  fast 
the  burden  of  a  great  hope,  only  the  patience  strong  and  meek 
which  is  content  to  bow  beneath  the  fatigue  of  a  long  and 
distant  purpose,  only  these  stepping  stones  and  no  gulf  im- 
passable to  human  feet  divide  aspiration  from  achievement."  It 
is  consoling,  too,  sometimes  to  reflect  that,  whether  one  succeeds 
or  whether  one  fails,  g,  man  cannot  do  more  than  his  level  best. 
As  Epictetus  said — "  It  is  not  for  the  actor  to  choose  his  part, 
but  it  is  the  actor's  duty  to  play  the  part  that  is  given  to  him  to 
the  best  of  his  ability,  the  result  being  left  to  the  director  of  the 
play."  And  that  the  crippled  philosopher  accepted  his  part  cheer- 
fully may  be  gauged  from  his  words — "  For  what  else  can  I  do, 
an  old  man  and  lame,  than  sing  hymns  to  God?  If  I  were  a 
nightingale,  I  would  do  after  the  manner  of  a  nightingale ;  if  a 
swan,  after  the  manner  of  a  swan.  But  now  I  am  a  reasoning 
creature,  and  it  behoves  me  to  sing  the  praise  of  God.  This  is 
my  task,  and  this  I  do,  nor,  as  long  as  it  is  granted  to  me,  will  I 
ever  abandon  the  post.  And  you,  too,  I  summon  to  join  me  in 
the  same  song." 


277 


INDEX. 


ACCUMULATORS,  Electric,  206. 

,,  Hydraulic,  212. 

Adamson  rings,  29. 
Air  pump,  Capacity  of,  169. 
Alloys,  8 
Alternating  current,  Periodicity  of, 

204. 

Alternators,  199. 
Aluminium,  8. 

,,  bronze,  10. 

Ampere,  195,  209. 
Analyses  of  gases,  249. 
Arc  lamps,  197. 
Area  and  circumference  of  circles,  265. 

,,    of  ports,  137. 


B 


BABBIT  metal,  10. 

Babcock  boiler,  39. 

Balanced  slide  valve,  93. 

Batteries,   Primary  and   secondary, 

•206. 

Beams,  Strength  of,  255. 
Bearings,  Pressure  on  engine,  139. 

Shafting,  158. 
Belleville  boiler,  40. 
Bellis  engine,  111. 
Belts,  Canvas,  151. 
,,      Leather,  149. 
Bessemer  steel,  7. 
Blake  boiler,  36. 
Boiler  compositions,  54. 
Feed-water,  54. 
flues,  44. 
Boilers,  27. 

Combustion  of  fuel  in,  51. 
Evaporation  of,  49. 
Galvanic  action  in,  37. 


Boilers,  Pitting  in,  38. 

Shell-type,  27,  et  seq. 

,,          dangers  of,  37. 

Steel  plates  for,  18. 

Strength  of,  41. 

Testing,  55. 

Water -tube,  39,  et  seq. 
Bolts  and  nuts,  19. 
Brake,  118. 
Brass,  10. 
Brazing,  8. 

British  thermal  unit,  47. 
Bronzes,  9. 

Browett-Lindley  engine,  111. 
Brush- Parsons  turbine,  181. 


CALORIFIC  value  of  coals,  50. 
Calorimeter,  56. 
Cameron  pump,  22. 
Capacity  of  pumps,  61. 
Carbon  in  iron,  4,  5. 

,,       in  steel,  5,  6,  7. 
Casehardening,  1. 
Castings,  Effect  of  cooling  on,  11. 
Cast  iron,  2,  17. 

„    steel,  5,  17. 
Castle  nuts,  20. 
Centrifugal  force,  131. 

,,  pumps,  228. 

Chimneys,  Area  of,  52. 

,,          Draught  in,  51. 
Chromium,  Effect  of,  on  steel,  6. 
Coal,  Calorific  value  of,  50. 

,,      conveyers,  69. 
Coefficient  of  friction,  139. 
Combustion.  Rate  of,  51,  53. 
Compound  engine,  102. 

,,  ,,      Calculation    of 

power  of,  115. 


278 


INDEX. 


Condenser,  Barometric,  174. 
„          Ejector,  173. 
,,          Evaporative,  173. 

Jet,  165. 

,,          Surface,  167. 
Connecting-rods,  138. 
Consumption  of  oil  and  petrol,  250- 

254. 
,,  ,,   steam    in    engines, 

103. 

Conveyers  for  coal,  69. 
Cooling  towers,  174. 
Copper,  8. 

,,       wire,  Strength  of,  15. 
Corliss  valve  gear,  104. 
Cornish  boilers,  30,  32. 
Corrosion  in  boilers,  37. 

,,         of  condenser  tubes,  171. 
Cost  of  materials,  18. 
Crank  shafts,  Material  for,  5. 

,,  Proportions  of,  139. 

Cruse  superheater,  67. 
Curtis  turbine,  185. 
Cylinder  walls,  Thickness  of,  137. 
Cylinders,  Iron  for,  3. 
,,          Ratio  of,  137. 


DECIMAL  equivalents,  264. 
De  Laval  turbine,  182. 
Delta  metal,  9. 

Diesel  oil  engine,  241,  243,  254. 
Dished  boiler  ends,  43. 
Dowel  pins,  20. 
Dowson  gas  plants,  246. 
Draught,  Induced,  52. 
,,        Natural,  51. 
Double-beat  valves,  108. 
Drop  valves,  108. 
Dry-back  boiler,  37. 
Duplex  pump,  57. 
Dynamos,  193. 


E 


ECCENTRIC-RODS,  138. 
Economic  boiler,  33. 
Economisers,  63. 
Edwards  air  pump,  172. 
Efficiency  of  gas  engines,  245. 

,,         ,,  steam  engines,  144. 
Ejector  condenser,  173. 


Elasticity,  Young's  modulus  of,  16. 
Elastic  limit,  12. 
Electrical  units,  196,  209. 
Electric  current,  Production  of,  193. 
Elongation,  Percentage  of,  12. 
Engines,     see    under     Steam,     Gas, 

Petrol,  and  Oil. 
Evaporation  of  a  boiler,  Calculation 

of,  49. 

,,  „  Cornish  boilers,  33. 

,,  ,,  Lancashire     boilers, 

32. 
Evaporative  condenser,  173. 

,,  power  of  coal,  50. 

Exhaust  pipes,  82. 
Expansion,  Governing  by,  134,  136. 
,,          of  pipes,  73. 


FACTORS  of  safety,  16. 

Fahrenheit  degrees,  Conversion  into 

Centigrade,  262. 
Falls  of  Foyers  turbine,  228. 
Fatigue  of  metals,  15. 
Feed  pumps,  57,  et  seq. 
Feed-water,  54. 

, ,  heaters  and  economisers, 

63. 

Filters,  Oil,  70. 
Flanges,  Table  of,  80,  81. 
Flash-point  of  oils,  254. 
Flow  of  steam  through  pipes,  74. 

,,      ,,  water  through  pipes,  235. 
Flues,  Strength  of  boiler,  44. 
Flywheel  pumps,  60. 
Flywheels,  129,  133. 

,,  Energy  stored  in,  133. 

,,  Form  of  arms  of,  11. 

Stress  in,  131. 
Forced  draught,  52. 
Foundation  bolts,  21. 
"  From  and  at"  explained,  49. 
Froude  water  brake,  120. 
Fuels,  Calorific  value  of,  50. 
Furnace,  acid  and  basic  lining,  7. 


GALLOWAY  boiler,  33. 
Galvanic  action  in  boilers,  37. 
Gas  engines,  239. 
,,   suction  plant,  246. 


INDEX. 


279 


Gas  threads,  23. 

Gearing,  Power  transmitted  by,  158, 

162. 

Worm,  159. 
Governing  reciprocating   engine  by 

expansion,  134,  136. 
,,  reciprocating   engine  by 

throttling,  134,  136. 
,,  steam    turbines,    Curtis, 

186. 
,,  ,,         intermittent, 

179. 
,,  ,,         throttling, 

181. 

Grate  area,  52. 
Grover  washer,  20. 
Gunmetal,  8. 


H 


HANS  REYNOLDS  chain,  162. 
Hardening  of  steel,  6. 

,,  ,,  wrought  iron,  1. 

Heat  lost  by  radiation  from  pipes, 

78. 

,,      Mechanical  equivalent  of,  47. 
Heating  surface  of  Cornish  boilers,  33. 
,,  ,,       ,,  Lancashire  boilers, 

32. 

Helical  wheels,  159. 
Helicoid  nuts,  20. 
Hornsby  oil  engine,  252. 
Horse-power,  96. 
Howden's  forced  draught,  69. 
Hydraulic  accumulator,  212. 
jack,  218. 
jigger,  218. 
lifts,  217. 
memoranda,  237. 
press,  211,  217. 
ram,  235. 
riveter,  216. 
testing,  219. 
Hyperbolic  curve,  99. 


IGNITION  for  gas  engines,  241. 
Impact  tests,  12. 
Incandescent  lamps,  197. 
Indicator  diagrams,  124. 
Indicators,  122. 
Induced  draught,  52. 


Injectors,  61. 
In  ward- flow  turbine,  221. 
Impulse  water  wheel,  225. 
Iron,  Cast,  2,  17. 

, ,     Malleable  cast,  4. 

,,     Wrought,  1. 
Isolating  valves,  88. 


JACKETING  steam  engines,  114. 
Jockey  pulleys,  152. 
Joints,  Expansion,  73. 

, ,       Lap  and  butt,  25. 

,,       Strength  of,  45. 
Joists,  Strength  of,  255. 
Jonval  turbine,  225. 
Joule's  mechanical  equivalent  of  heat, 

47. 
Journals,  Pressure  on,  139. 


K 

KO'RTING  gas  engine,  243. 


LANCASHIRE  boiler,  27. 
Lap  of  slide  valve,  91. 
Lead  of  slide  valve,  92. 
Lentz  valve  gear,  108. 
Lewis  bolts,  '21. 
Lighting  by  electricity,  197. 
Lock  nuts,'  20. 
Locomotive  boiler,  33. 


MALLEABLE  cast  iron,  4. 
Manganese  bronze,  9,  17. 

,,          Effect  of,  on  steel,  6. 
Marine  boiler,  36. 
Materials,  Cost  of,  18. 
,,          Testing,  11. 
Mechanical  equivalent  of  heat,  47. 

,,  stokers,  67. 

Meldrum's  forced  draught,  69. 
Metallic  packing,  141. 
Mild  steel,  2,  5,  17. 
Modulus  of  elasticity,  16. 


280 


INDEX. 


Mond  gas,  249. 
Muntz  metal,  10. 


N 


NIAGARA  FALLS  turbines,  228. 
Nickel  steel,  5,  8. 
Niclausse  boiler,  40. 
Nuts,  19. 


OHM,  209. 
Oil  engines,  252. 
„  filters,  70. 
Open-hearth  steel,  7. 
Otto  cycle  gas  engines,  239. 
Outward-flow  turbines,  221. 


PACKINGS  for  steam  engines,  140. 
Parsons  steam  turbine,  177. 
Petrol,  Calorific  value  of,  254. 

,,       Consumption  of,  254. 

,,       engines,  252. 
Petroleum,  Calorific  value  of,  50. 
Phosphor  bronze,  9. 
Phosphorus  in  steel,  Effect  of,  8. 
Pig  iron,  Qualities  of,  3. 
Pipe  flanges,  79-81. 
Pipes,  Steam,  71. 

„     Exhaust,  82. 

,,     Expansion  of,  73. 

,,     Flow  of  steam  in,  75. 

,,         ,,     ,,  water  in,  235. 

,,     Size  of,  74. 

,,     Strength  of,  78. 

,,     Water  hammer  in,  81. 
Piston  rings  and  springs,  142. 

„      rods,  138. 

,,      speeds,  140. 
Pitting  of  boiler  plates,  38. 
Ports,  Area  of,  137. 
Power,  Calculation  of  engine,  96,  113, 
244. 

,,      Loss  of,  in  transmission,  155. 

,,      transmission  by  belts,  149. 

„  gearing,  158. 

,,  ,,  ,,  ropes,  153. 

,,  shafts,  156. 


Pressure  and  temperature  of  steam, 

48. 

,,       on  bearings,  139. 
Priestman  oil  engines,  252. 
Primary  battery,  206. 
Producer  gas,  247. 
Proportions  of  steam  engines,  137. 
Pulleys,  Convexity  of,  151. 

,,        Distance  apart  of  centres, 

152,  154. 

Fast  and  loose,  153. 
Form  of  arms,  11. 
Grooves  for  ropes,  154. 
Jockey,  152. 
Pump,  Cameron,  60. 

capacities,  61,  237. 
Deane,  60. 
Duplex,  57. 
Pulsometer,  233. 
valves,  252. 
Weir,  58. 
Pumps,  Materials  for,  235. 


QUALITY  of  metals,  16. 


RADIATION  from  pipes,  78. 
Ramsbottom  rings,  142. 
Rateau  turbine,  188. 
Ratio  of  engine  cylinders,  137. 
Reduction  of  areas,  12. 
Reversing  gear,  92,  95. 
Rivets,  24. 

,,  Strength  of,  45. 
Rolled  steel  joists,  258. 
Rolling,  effect  on  strength  of  steel, 

&c.,  15. 
Rope  driving,  153. 

,,       Pulleys  for,  154. 
Rotary  converter,  206. 


SAFETY  valves,  30. 
Scavenging  gas  engine  cylinders,  242. 
Set  screws,  19. 

Shafting,    Power   transmission    by, 
156, 


INDEX. 


281 


Siemens-Martin  open-hearth  steel,  7. 
Siemens  open-hearth  steel,  7. 

,,        producer  gas,  246. 
Skew  wheels,  161. 
Slide  bars,  Pressure  on,  139. 
,,     valve,  Balanced,  93. 
„      D,  91. 
,,      Piston,  91. 
Slip  in  pumps,  61. 
Specifications  as  to  quality,  17. 
Specific  gravity  of  oils,  254. 

,.       heat,  176. 
Speed  of  belts,  150. 

,,  gas  engines,  246. 

,,          pistons,  140. 
,,  ropes,  153. 

,,  turbines,  181. 

,,          Willans  engines,  110. 
Squares  and  cubes,  268. 
Steady  pins,  20. 
Steam  domes,  30. 

,,      engine  parts,  Proportions  of, 

137. 

,,      engines,  89. 
,,  ,,       Consumption,  103. 

,,       Efficiency  of ,  144. 
,,  ,,       Proportions  of,  137. 

„       Testing,  116. 
,,      Latent  heat,  47. 
,,      pipes  and  valves,  71,  et  seq. 
,,      Properties  of,  46. 
,,      -raising  accessories,  57. 
,,      Saturated,  49. 
Specific  heat,  176. 
Superheated,  49,  66,  78. 
Table  of,  48. 
traps,  83. 
turbines,  177. 
Steel,  2,  5,  17. 

,,      Hardening  and  tempering,  6. 
,,      High-speed  tool,  6. 
,,      Self -hardening,  6. 
Stirling  boiler,  41. 
Stokers,  Mechanical,  67. 
Stone's  bronze,  9. 
Stop  valves,  72,  77,  85,  86. 

,,          ,,       Obstruction  of,  77. 
Strength  of  beams,  255. 
boilers,  41. 
bolts  and  nuts,  23. 
cylinders,  78. 
materials,  1-15. 
riveted  joints,  45. 
Stress  and  strain,  12. 
Studs,  19. 


Suction  gas  plant,  246. 
Superheated  steam,  49,  78. 
Superheaters,  66. 
Surface  condensers,  167. 


TEETH  of  wheels,  159,  163. 
Temperature,  effect  on  metals  and 

alloys,  16. 
Testing  boilers,  55. 
,,       elongation,  12. 
,,       impact,  12. 
,,       machines,  11-13. 
,,       materials,  11;  see  also  Tables 

I.  and  II. 

,,       steam  engines,  116. 
Thermal  efficiency,  145,  245. 
,,        storage  system,  65. 
Thornycroft  boiler,  41. 
Threads,  Whitworth  and  gas,  22,  23. 
Throttling,  Governing  by,  134,  136. 
Transmission  of  heat,  54. 

,,  „  power,  149. 

Tungsten,  Effect  of,  on  steel,  6. 
Turbines,  Economy  of,  190. 

Steam,  177. 
„         Water,  221. 


U 


UNITED  STATES  packing,  141. 


VACUUM  augmenter,  171. 
Valve,  Balanced,  93. 

Corliss,  104. 

D-form,  89. 

Drop,  108. 

Hopkinson-Ferranti,  87. 

Isolating,  88. 

Lentz,  108. 

Piston,  91. 

rods,  138. 

Stop  and  gate,  72,  85,  86. 
Valves  and  pipes,  71. 
Van  der  Kerchove  engine,  108. 
Vertical  boilers,  37. 
Vibration  of  engines,  111. 
Volt,  195,  209. 

19 


282 


INDEX. 


w 

WATER  brake,  Froude,  120. 

hammer,  81. 

-tube  boilers,  39. 

turbines,  221. 

Weight  of,  237. 

wheels,  220. 
Weir  pump,  58. 
Welding  heat,  1. 
Westinghouse  turbine,  188. 
Wheels,  Raw-hide,  161. 
„        Toothed,  159. 
,,        Worm,  159. 
White  metal,  10. 
Whitworth  compressed  steel,  5. 

„          threads,  22. 
Willans  engine,  108. 


Willans- Parsons  turbine,  180. 
Wo  bier's  law,  16. 
Worm  gear,  159. 
Worthington  pump,  57. 
Wrought  iron,  1. 


YARROW  boiler,  41. 

Young's  modulus  of  elasticity,  16. 


ZEUNER  diagram,  145. 
Zoelly  turbine,  189. 


OF   THE 

UNIVERSITY 

OF 


BELL  AND   BAIN,    LIMITED,    PRINTERS,   GLASGOW 


IS 


53813 

- 


